The Twin Fighters

© Emmanuel Gustin, 2004-2006

Few categories of aircraft have been so controversial as the propeller-driven, twin-engined fighter aircraft of the 1930s and 1940s. Some designs in this category, such as the Lockheed P-38 Lightning, still have ardent admirers, as well as convinced detractors. Other types, such as the Messerschmitt Bf 110, are routinely dismissed as failures, despite their historically very important role. On the whole, the judgement of most aviation enthusiasts about the twin-engined fighters has been remarkably negative. Historians tend to be more moderate, but most of them still tend to dismiss the category as relatively unimportant.

The controversy never died, because the facts allowed everyone to have a little bit of truth. The aircraft in this category were very diverse, with a wide range in capabilities corresponding to some very different intended roles. Still, most people tend to judge the twin-engined fighter on its ability to match the more glamorous single-engined fighter in the very roles in which the latter excelled: Daylight air superiority, interception and escort. This, inevitably, puts the heavier and less manoeuverable twin-engined types in an unfavourable light. A few could indeed compete with single-engined fighters in these roles, but even in these cases arguments of economy and efficiency usually favoured the single-engined fighter. Seen in this light, the “twin” appears to have been a pointless aberration.

Nevertheless many thousands of these aircraft were built, to a substantial number of different designs. And while some were clear failures, others served with great distinction, as long-range fighters, ground attack aircraft, nightfighters, bomber interceptors, anti-shipping aircraft and reconnaissance aircraft. When given missions that corresponded to the strengths of the twin-engined designs, rather than their weaknesses, they proved highly useful and successful; even if their performance parameters were sometimes unimpressive compared to those of the single-engined thoroughbreds.

Ancestry: The Great War

The first moderately successful use of large, twin-engined fighter aircraft can be attributed to the French during the First World War. The British conducted numerous experiments with multi-engined fighters, but with a distinct lack of success. The German and Austro-Hungarian air forces showed relatively little enthusiasm for the concept. The German production of large long-range bombers was not paralled by a similar development of large long-range fighters to escort them.

The more economical solution, which usually prevailed during the First World War and remained popular after it, was the single-engined two-seat fighter. This could fly many of the missions of the larger multi-engined type, demonstrated great operational flexibility, and was considerably less expensive. Such aircraft had fixed forward-firing guns and one or two flexible weapons for the rear gunner. Their manoeuverability was reasonably close to that of the single-seaters, and their performance, in an era of drag-inducing struts and wires, was not much inferior. The crews of the Bristol F.2 'Brisfit' demonstrated that the best way to operate a good two-seater was to fly it in combat as a single-seater, using the fixed guns as primary armament.

France

In March 1915 Caudron derived the twin-engined G.4 from the single-engined G.3, in response to a requirement for an Army co-operation aircraft armed with a machine gun. Instead of one 80 hp Le Rhône engine in the nose, two such engines were installed between the wings. A gunner was seated in the nose of the fuselage, with a single 7.7 mm Lewis or Hotchkiss machine gun. The G.4 was larger and heavier than the G.3, with a wing area of 36.8 square meter against 27 square meter; top speed was increased from about 110 km/h to 130 km/h. The G.4 was used mostly for army co-operation or bombing missions, but escadrille C 66 flew long-range escort missions for the Voisin bombers of GB 2, and some other units followed this example.

By early 1916 the G.4 was already obsolescent. Initial attempts to find a replacement focused on the A3 category, defined as a twin-engined three-seat aircraft that would be able to fly long-range reconnaissance, bombing, and escort fighter missions. A number of aircraft were built to meet this specification, but none of them was suitable as a fighter. In November 1916 a C3 requirement for a specialized three-seat escort fighter emerged. The specifications called for an aircraft with a top speed of 170 km/h at 2000 m, able to climb to 3000 m in less than 15 minutes. It would have to be able to carry two machine guns, with 500 rounds each, and light armour.

Whether the Caudron R.11 was designed to meet the C3 or A3 specification is unclear, but eventually this was the type that was employed as a long-range escort fighter. With two 215 hp Hispano-Suiza 8Ba engines, the R.11 met the C3 specifications, flying at 183 km/h at 2000 m and climbing to 3000 m in 14 minutes and 30 seconds. It carried five 7.7 mm Lewis guns, in two flexible twin mounts on the nose and aft fuselage, plus a single flexible gun mount in the lower nose, to be used by the nose gunner. However, only 54 are reported to have been in service at the end of the war, because the type arrived late and was further delayed by engine problems. Of the original 1000 ordered, only 370 were completed.

Their success was debatable. Initially, the R.11s flew ahead of the bomber formations to take on the German fighters. Later tactics saw the R.11s flying close escort among the bombers, contributing to the firepower of the formation with their armament, while single-seat fighters provided top cover. The crews of the R.11s had a strong bond with the bomber crews they protected, because every R.11 escadrille was assigned to provide cover for a specific bombardement escadre. (Such an escadre typically contained two or three groupes de bombardement, each containing three escadrilles.) But when the top cover of SPAD or other fighters was not present, the combined formation of bombers and R.11 escorts still proved vulnerable to enemy fighter attacks.

Two variations had entered tests at the end of the war, but were abandoned after the Armistice. The R.12 was an R.11 with 300 hp Hispano-Suiza 8Fb engines. The R.14 was a larger development of the R.11 designed to meet the Ca3 requirement for a three-seat fighter armed with a 37-mm Hotchkiss cannon. It was powered by the same 8Fb engines as the R.12, and retained the five machine guns in addition to the cannon.

There were other French experiments with multi-engined fighters during WWI, with besides the escort mission also a putative “anti-airship fighter” role driving such projects. But they remained far removed from the level of performance and success achieved by the R.11. The Letord 6, for example, was a development of the Letord 3 night bomber with a 37-mm cannon in the nose, to meet a Ca3 requirement for a cannon-armed escort fighter. Considered inferior to the Caudron R.14, it was abandoned.

Great Britain

In Britain too interest in the twin-engined fighter concept arose early. In 1915 Vickers flew the F.B.7 and F.B.8. The F.B.7 was a twin-engined biplane designed to accomodate a nose gunner with a 1-pounder cannon in the nose. This was an ambitious concept, but with two 80 hp Renault engines the top speed was a disappointing 121 km/h. Vickers convinced the RFC that the smaller F.B.8, powered by 100 hp Gnome radials and armed only with a Lewis machine gun, was a better concept; but its performance was still judged insufficient.

Of course, these aircraft were designed at a time when there was little knowledge about the best configuration for a fighter aircraft, and for a time the F.B.7 and F.B.8 must have seemed to have as much potential as the F.B.5 single-engined pusher biplane and its developments. Experimentation continued and a number of twin-engined fighter prototypes were built and flown, only to be rejected for service. Even with the benefit of hindsight, one can hardly dispute the wisdom of those decisions.

The Avro 523 Pike, first flown in March 1916, was designed for the Admiralty as a long-range escort and anti-airship fighter; it had two 150 hp pusher engines and single Lewis guns fore and aft. It did not enter production, despite attempts to develop it as a night bomber.

The Bristol T.T.A (Twin Tractor Model A), following one month later, was intended as a defensive fighter, with two flexibly mounted Lewis guns in the nose. (A cannon was considered as alternative armament.) The pilot, seated well aft, was supposed to use a flexible Lewis gun for the rear defense, as well as fly the aircraft. Powered by two 120 hp Beardmore engines, the T.T.A made an unfavourable impression on test pilots. Only the two prototypes were completed.

The Pemberton-Billing P.B.29E and P.B.31E were dedicated anti-airship fighters, designed to mount standing patrols at night and engage the enemy with their flexible armament. These were large, lumbering aircraft; the P.B.29E was a quadruplane powered by two 90 hp Austro-Daimler engines, with a gunner in a nacelle mounted between the upper pair of wings, while the fuselage was between the lower pair of wings. On the improved, more streamlined P.B.31E the gunner's pulpit was faired into a repositioned fuselage. The P.B.31E could boast an endurance of 18 hours on patrol and an armament of one 1 ½-pounder recoilless cannon and two machine guns, but was too slow and underpowered to intercept an enemy airship. By 1917, when the P.B.31E flew, the threat was already shifting to enemy bombers, and these outperformed the quadruplane by a considerable margin.

US Navy

At the end of the war, the Allies formulated a requirement for an aircraft that could protect long-range flying boats, such as the Curtiss H-16 and Felixstowe F.5. The U-boat patrols flown by these aircraft were essential, but over the North Sea they risked interception by German fighters. Although the specification for a long-range escort fighter was prepared by the British government, the task of actually designing and building it was handed to the Naval Aircraft Factory, an aircraft design and production facility that was controlled by the US Navy.

The war was over before construction work could begin. The TF or Tandem Fighter made its first flight in October 1920, powered by two 300-hp Hispano-Suiza engines, installed in tandem between the biplane wings. The configuration of the TF was that of a flying boat with a short hull, its tail surfaces lifted high over the water by an arrangement of struts. The engine installation caused no end of trouble, until in 1923 it was finally realized that any usefulness the TF might have possessed, had evaporated long ago.

The Central Powers

One German venture in the field of the twin-engined fighter is worth mentioning because it pioneered the concept of the ‘centreline thrust’, in which the fuselage contains a tractor and a pusher engine. This avoids power asymmetry in the event of an engine failure, and allows two engines to be installed in a smaller airframe than would necessary if the engines were on the wings.

The Siemens-Schuckert DDr.I was a triplane with relatively large gaps between the wings; a nacelle attached to the central wing had a 120 hp Siemens-Halske Sh.I engine fore and aft. The DDr.I was a single-seat fighter with armed with two synchronized LMG 08/15 machine guns. The tail was carried on a cage of struts around the rear propeller, as on most WWI pusher types; to accomodate this the pusher propeller was four-bladed and of smaller diameter than the two-blade tractor propeller. In November 1917 a single test flight was made, which ended in a crash. The plan to develop a more powerful DDr.II was abandoned.

In Austria-Hungary, a ‘battleplane’ program was launched in August 1914. The specification called for a twin-engined aircraft armed with two machine guns, and protected by 120 kg of armour plate. In November formal orders were placed with Aviatik, Albatros and Lloyd to develop their designs further. But by December 1916, officers had come to the inevitable conclusion that the ‘battleplane’ concept did not correspond to valid a military role, and so all production orders were cancelled. Only a few prototypes were flown.

Of these the Lohner 10.21, flown in April 1916, was a fairly conventional biplane with two 150 hp Daimler tractor engines, strongly swept back wings, and unsatisfactory performance. It had a front gun position with room for two gunners, and a single gunner in a rear position. The Phönix 20.10 was flown in September 1916, also a conventional biplane with two 200 hp Hiero engines. After cancellation of the battleplane program it was suggested that this type might be developed as a trainer, but no more 20.10s were built. The innovative Lloyd 40.06 tried to achieve streamlining by burying the two 160 hp Daimler engines in the front fuselage, driving a common gearbox, from which two tractor propellers were driven. Development of the gearbox was identified as a problem early on, and in fact this was never completed, preventing assembly of the 40.06. Engineering work was nevertheless allowed to continue until June 1917, more out of interest in the gearbox than in the aircraft.

Interbellum: The battleplane

After the end of the Great War, several influential theorists formulated their vision on the use of combat aircraft. They based their opinions both on their experience in the previous conflict and on their imaginative view of the future, with perhaps an overly strong bias towards the latter. The most famous was the Italian Giulio Douhet, who advocated an unrestrained air offensive. In his theories there was little room for fighter aircraft, because these were seen as elements of a defensive strategy that he rejected. Instead, the domination of the air was to be ensured by offensive aircraft, i.e. bombers, whose performance and heavy armament would make any attempt at defense futile. Adherents of this theory were induced to design “battleplanes”, well-armed aircraft that would be able to bomb as well as fight for air superiority.

For many years, this remained little more than theory. Stocks of war materiel were plentiful and defence spending limited. During the 1920s, design teams remained active, and some quite innovative aircraft were produced; but they met little interest from their potential customers. For the most part, air forces were content with designs that were fundamentally similar to those of 1918, although they replaced wood by metal, which was lighter and easier to maintain, and installed more powerful and reliable engines. In the 1930s, under the strong influence of innovations in civilian aviation and a worsening of the political situation, change could no longer be avoided, and then it became both sudden and radical.

France

In March 1933 the French government decided to entirely renew the organisation and equipment of its air force. An independent air force, L'Armée de l'Air, was created on the first of April. Officially, an offensive air strategy was adopted, but actually 56% of the available aircraft were equipped for army co-operation or reconnaissance, while only 16% were bombers and 28% were fighters. To increase the offensive power of the air force, it was decided to both reduce the share of reconnaissance squadrons and to give them aircraft with more offensive ability, so-called BCR types. The acronym stood for Bombardement – Chasse – Reseignment, or Bomber – Fighter – Reconnaissance.

This, of course, was only a few months after Hitler came to power in Germany. Contrary to what is often assumed, the French government was well aware of the need to build a modern and powerful air force to counter the growing threat at the other side of the Rhine, but there was a deep gap between the series of increasingly ambitious re-equipment plans (calling for first for 1023, and then 1554, 2617, 2938 and finally over 9000 aircraft in the first line) and the meagre capability of the industry to develop new aircraft and mass-produce them. Frustrated, Pierre Cot, Minister of Air between 1936 and 1938, nationalized most of the aircraft manufacturers and regrouped them into larger entities. After much initial confusion this did some good, but it was only a half-measure – the production of aircraft indeed increased but the production of engines and instruments (which had not been reorganized, because of budgettary restrictions) failed to follow.

In July 1934 the French parliament approved the so-called Plan I, also called the Plan of 1023, because it called for 1023 combat aircraft in the first line and 342 in reserve. This ambitious plan had two major flaws: The aviation industry was incapable of building the required aircraft (by the end of 1935 it would have delivered much less than half the aircraft desired) and it relied far too much on the success of the BCR category. This became a serious disappointment indeed and only one real BCR entered service, the Potez 540. The BCR requirement of October 1933 called for a top speed of 350 km/h and a cruising speed of 250 km/h. This was modest enough, but unfortunately most of the design teams appeared to be participating in a dubious contest for aeronautical ugliness, to a degree that was highly incompatible with efficient aerodynamics. In fact the armament that was supposed to allow these aircraft to win a clash with enemy fighters only amounted to a few machine guns extra, yet the desire to create gun positions with a good field of view and fire resulted in some very awkward constructions. In 1934 L'Armée de l'Air finally decided that nose gun turrets should be omitted from the BCR designs, in an attempt to reduce drag.

Of course, even in 1933, the idea was far from new; specifications for a multiplace de combat or multi-seat combat aircraft had been released regularly since the early 1920s. A radically innovative effort in this direction started as early as 1922, when a team at Blériot conceived a monoplane successor to the Caudron R.11. In 1923 it offered the Blériot 117, a wooden monoplane with gunnery positions in the nose and at the rear end of both engine nacelles, which had been deepened and elongated for the purpose. Two years later this had evolved in the model 127, a good performer for the time with a top speed of 221 km/h at 4000 m, equal to that of fighters. It was declared the winner of the 1926 multi-seat combat aircraft competition, and 34 were ordered to equip two “protection squadrons” within the 11th Escadre de Bombardement. But the success story ended abruptly after a series of fatal accidents. An investigation revealed that the turbulence generated by the deep wing root, the large engine nacelle and the gunner's position caused the small tailplane to lose effectiveness, with fatal results. The aircraft was definitively grounded.

To solve the aerodynamic problems of the 127, the entirely redesigned Blériot 137 was flown in 1930. This aircraft was of all-metal construction. The new wing and tailplane were located higher, and the rear gunners were no longer at the rear of the engine nacelles, but in cylinderical bulges attached to the side of the fuselage. Unfortunately, official tests revealed that this disturbed the flow over the tail even more than the original solution: The 137 entered a steep vertical dive if the engines were throttled back and remained uncontrollable until power was restored! It has been claimed that Blériot's test pilots had discovered this before the 137 was submitted for official testing (as indeed they should have), and the company had chosen to keep this secret and simply restricted the throttle travel! Whether this story is true or not, the firm might as well not have bothered, because eight of these totally useless aircraft were ordered anyway. Officially this was a compensation for the replacement of the 1928 multiplace de combat programme by the new BCR programme, which would have ruled out an order for the 137 regardless of the results of the test programme.

The SPCA 30 M of 1931 was a fairly elegant cantilever low-wing monoplane, of all-metal construction with corrugated metal skinning. Gunnery positions were located in the nose and in each of the twin tail booms. The 30 M was big, with a wing span of 25.60 m and a maximum take-off weight of 7200 kg; even the wheels of the fixed landing gear stood as tall as a man. With two 650 hp engines its top speed was 255 km/h, and its flying qualities reportedly left much to be desired.

The Blériot and SPCA designs look awkward today, but they were models of streamlining in comparison to the extraordinary Breguet 413. This was a twin-engined biplane with a short, deep and angular front fuselage, and a slender tail boom to create a good field of fire for the rear gunner. The 413 looked as if it had come from the drawing board of a designer of toys instead of real aircraft. It was armed with six machine guns: Two each in a nose turret, dorsal gun mount and ventral turret. First flown in February 1933, and powered by two 650 hp Hispano-Suiza 12Ybrs engines, it was capable of 310 km/h. (The winner of a contemporary competition for a single-seat fighter, the monoplane Dewoitine D.500, reached 367 km/h at 5000 m.) This bizarre aircraft was about to enter production, for 50 had been ordered under the Plan I, when an accident with the prototype led to cancellation.

The Amiot 140, also designed for the 1928 M4 requirement, was at least superficially a much more impressive design, because it was a cantilever monoplane with smooth stressed-skin construction and, in prototype form, fairly clean lines. But the streamlined appearance was deceptive; it had a very thick wing with an enormous area (adopted to keep the landing speed low), and the front fuselage had been designed as a two-decked structure of which only the upper deck faired in with the tail. The lower deck provided a location for the lower rear gunner, ample viewing windows for the observation mission, and even back-up flying controls. Top speed was only 242 km/h at sea level. The open cockpit and ditto gunnery position in the nose were already very uncomfortable at such speeds, and the production Amiot 143 had an enclosed cockpit and nose turret; the dorsal and ventral positions remained open but were better faired in. With 800 hp radial engines, the aircraft then reached 293 km/h at 4000 m. By the standards of 1933, the Amiot 143 could not hide its obsolescence, but nothing better was available, so the aircraft was ordered as a bomber.

To create a competitor for the 1933 BCR requirement, Amiot suggested a cleaned-up 143 with a smaller wing and retractable landing gear. On paper the Amiot 144 met the BCR specification, but when it the prototype was tested in 1936 the results were disappointing. Amiot persisted in calculating the hypothetical performance with equally hypothetical engines of 1200 hp or even 1500 hp, but the air force abandoned the 144 and ordered a few more 143s instead.

The Potez 540 and its derivatives finally emerged as the winners of the BCR orders. It was a high-wing strut-braced monoplane with engines carried in nacelles below the wings, a concept that was rather old-fashioned, although it did allow the main landing gear to retract in the rear of the engine nacelles. It had nose, dorsal and ventral turrets, each with a single 7.5 mm Darne machine gun. The top speed of 310 km/h already fell considerably below the mark for a good multi-role aircraft. When the Spanish Civil War broke out, a number of Potez 540s were delivered to the Republicans, but the combat experience with these obsolescent aircraft was poor.

Marcel Bloch, later to become more famous as Marcel Dassault, designed the Bloch M.B.130. This was a streamlined, elegant twin-engined monoplane (not unlike the later Martin Maryland), but it did not have a performance commensurate with its slender lines. It was first flown in June 1834. The further developed M.B.131 was, with two 880 hp Gnome-Rhône 14No radials, capable of 400 km/h. It was ordered by the air force, but as light bomber and reconnaissance aircraft. The fighter role was abandoned, and armament restricted to three 7.5 mm MAC 1934 machine guns and a small bomb load. Pictures do exist, however, of a M.B.131 with a much more substantial weapon, difficult to identify, in the nose.

Britain

During WWI, Britain had been the target of German bombing attacks, first by airships, and then by German long-range bombers. After attacks in daylight proved too costly for the unescorted bombers, the offensive switched to night bombing. The best night fighters of the period were slightly modified day fighters, and their effectiveness was quite low. It is logical, therefore, that after the war a specification 4/24 was formulated for a twin-engined home defence fighter, intended to defend the country against night bombers. A few months after its publication, in September 1925, the Air Ministry informed the contenders of a requirement to install two 1½ pdr COW guns, as well as supercharged engines. A top speed of 200 km/h (125 mph) was required, and a landing speed of no more than 80 km/h (50 mph). Although the 37-mm COW cannon was light for its calibre, it was bulky. Therefore such an aircraft would necessarily be rather large and could be expected to have a less than brilliant performance.

Two designs were offered. Bristol's Type 95 Bludgeon, later renamed Bagshot, was superficially the more advanced of the two, because it was an all-metal, high-wing, semi-cantilever monoplane. Or perhaps it ought to be called a sesquiplane, because there was an aerofoil of considerable size between the fuselage and the main wheels. It had gunner's positions in the nose and behind the wings, while the pilot sat close to the leading edge of the wing; all the cockpits were open. In addition to the heavy mounting points for the COW gun, there was a Scarff ring for a machine gun in the rear cockpit. Bristol actually calculated that the Bagshot would not meet the requirements, but the Air Ministry insisted on ordering a prototype. This revealed structural problems on its first flight, and was not flown again.

The competing Westland Westbury was a traditional biplane design, and as an aircraft it was considerably more successful, although it too failed to meet the performance targets. As the RAF had quickly abandoned the concept of the large, slow nightfighter, only the two prototypes were completed. They were used extensively for armament trials at Martlesham Heath, and fired large numbers of rounds at air and ground targets, reportedly not without some structural damage due to the recoil of the cannon. Besides the COW gun positions fore and aft, the Westbury had a ventral Lewis gun and a fixed Vickers. As on the Bagshot, the rear cannon had limited mobility on a simple trunnion mounting, but a complex mounting allowed a wide field of fire for the nose gun.

A much more interesting experiment was the Boulton & Paul Bittern, built to specification 27/24 for a twin-engined, single-seat nightfighter. The Bittern was a semi-cantilever, shoulder wing-monoplane. The choice for a twin-engined monoplane nightfighter was definitely a radical one, but logical enough, for a nightfighter needed the best possible view for the pilot, so it made sense not to obstruct it with an engine in front of him or an upper wing above him. Unfortunately, problems with wing and aileron flutter delayed the Bittern, and a comparison of early and later photographs shows that the problem was solved by the addition of extensive struts under the wing, which marred the clean lines of the aircraft and can have done little for its performance. With two small 230 hp Armstrong-Siddeley Lynx radials, the Bittern was underpowered. The second and last prototype incorporated an interesting experimental armament: Vertically swivelling Lewis guns in barbettes at the sides of the fuselage, coupled to a frame around the cockpit that carried the gunsight and moved synchronously.

Another twin-engined fighter design resulted from specification F.7/30 for a new RAF day fighter. Most designs to this specification were single-engined, but uncoventional designs were being encouraged by the RAF, which stressed the need for higher performance and a better field of view than offered by the old biplanes. Boulton-Paul offered the P67 design, an interesting mix of the old and the new. It was a low-wing, braced monoplane with an open cockpit and retractable landing gear, powered by two 395 hp Napier Rapier air-cooled, 16-cylinder in-line engines. A top speed of 365 km/h was calculated for this design. The RAF did not accept it, and it was never built.

Zerstörer

In 1932 specifications were prepared for the next generation of aircraft for the secret German airforce. They included a single-seat and a two-seat fighter, both single-engined. The two-seat fighter would be a multi-role type, suitable for air combat, ground attack, day and night reconnaissance, light bombing, and the release of chemical weapons. However, in January 1933 Hitler came to power, and subsequently the Reichs Aviation Ministry (RLM) was created, with Hermann Göring at its head. Before the end of the year the re-armament programme was revised.

The single-engined, two-seat fighter of 1932 was replaced by a twin-engined ‘Zerstörer’, with a crew of 3 or 4. The actual role of this substiantially larger aitcraft was only loosely defined, but it was primarily a fighter; long-range reconnaissance was to be a secondary task. A maximum speed of 400 km/h at 6,000 m was called for, and the ability to reach this altitude in 15 minutes. Range was to be 2000 km at the 330 km/h cruising speed. Landing speed was restricted to no more than 100 km/h, which had important implications for the wing area and loading. Crucially, there was no detailed specification of the armament, so the designers were left to choose for themselves how to install the two 20-mm cannon and two machineguns required. Presumably, one or more turrets were expected.

The largest of the contenders for the order was the Focke-Wulf Fw 57. With an empty weight of 6,800 kg and a wing span of 25 m, this was close in size to a medium bomber. It was intended to be powered by two DB 600 V-12 engines, but Rolls-Royce Buzzards were installed instead for the first flights, because the DB 600 was not yet ready. The flat nose of the Fw 57 was intended to carry a pair of 20-mm MG-FF cannon on semi-flexible mounts, i.e. with a restricted range of movement, while a Mauser-developed dorsal turret would contain a third cannon. It flew in early 1936, without the armament (which was never installed) but the performance of this overweight and underpowered aircraft fell well below the requirements. Only the three prototypes were completed.

The Henschel Hs 124 was lighter and smaller, with an empty weight of 4,250 kg and a wing span of 18.20 m. The three prototypes were completed to different configurations: The first one carried a mock-up of a planned nose turret with a Rheinmetall-Borsig cannon, and was powered by Jumo 210C engines. The second example had BWM 132Dc radial engines, and the nose turret was replaced by a glazed nose with a vertical slot, intended for the installation of two 20-mm cannon. The third prototype was completed as a two-seat aircraft with two 20-mm and two 7.9-mm guns fixed in the nose; the intended DB 600 V-12 engines had to be replaced by BWM 132 radials. The Hs 124 left a rather better impression than the Fw 57, but any series production was prevented by the success of the Bf 110.

As in the case of the Bf 109, Willy Messerschmitt ignored large parts of the official specification with the Bf 110. The Bf 110 was the smallest of the contenders, with a wing span of 12.6 m. It was basically a two-seat design, although a third crewmember could be seated if required. There was a single defensive gun at the rear of the cockpit canopy, but all other armament was to be fixed. This helped to reduce the size and weight of the aircraft and, given the limited power of the available engines, increased its performance.

The third prototype was the first to carry any armament, four fixed 7.92-mm MG 17 machine guns in the upper nose. On production aircraft, two MG-FF cannon were installed in the lower fuselage, firing through long blast tubes that ended at the lower side of the nose. This put the breeches of the cannon and the ammunition magazines behind the pilot's seat. The second crewmember operated a single MG 15 machine gun to defend the rear.

The prototype had DB 600 engines, but these were not available for the production B-0 and B-1 series. With Jumo 210D engines, performance of the B-series fell well short of that of the prototypes. But the Daimler-Benz DB 601A was installed in the Bf 110C, D and E series from late 1938 onwards, and with these engines, the Bf 110 could fly at 510 km/h at 5,000 m altitude.

There was, however, a competing design that promised to achieve higher performance. At Focke-Wulf, the disappointing results of the twin-engined Fw 57 and the single-engined Fw 159 (a parasol-wing aircraft that competed with the Bf 109) inspired Kurt Tank to design a radical, small twin-engined fighter. There was no official requirement for such an aircraft, but the RLM was sufficiently impressed to write a contract around Focke-Wulfs specifications in late 1935. The first flight of the new Fw 187 was made in April 1937, and it proved to be faster than the contemporary Bf 109B.

However, this Fw 187 did not qualify as a Zerstörer. Officially, it was classified as a light fighter, being single-seat and armed only with two MG 17 machine guns. As such it competed with the Bf 109 rather than the Bf 110, and the small speed advantage over the Bf 109, and the larger advantage in range, hardly seemed to justify the cost of two engines. The RLM now desired a second crew member, and the addition of two 20-mm MG-FF cannon to the fixed armament. Two prototypes were completed in the two-seat configuration with Jumo 210 engines, and two more with DB 600 engines. The price of such modifications was that the loaded weight of the small fighter rapidly rose, while range decreased.

Only three Fw 187A-0 pre-series aircraft were completed. By all accounts, the pilots who flew them on operations in Norway during the winter of 1940 were greatly impressed by these fast aircraft. Armament consisted of four fixed MG 17s, installed in the sides of the fuselage just below the sill of the cockpit canopy, and two MG-FF cannon in the lower fuselage. But the Luftwaffe still preferred the Bf 110, which was larger and roomier, and therefore more adaptable to other tasks.

Even more radical concepts were being developed at Arado. From 1935 onwards, designer Walter Blume worked on a twin-engined, twin-boom aircraft with large, streamlined gun turrets on the upper and lower side of the central nacelle. These would have a free field of fire towards the front and rear, because of the absence of a central tailplane between the booms: The horizontal tails were to extend to the outside. By 1937 this had evolved in a project E 500, which was to have two powered gun turrets with two 20-mm Rheinmetall-Borsig Lb 202 cannon each. The RLM initially showed some interest, but soon lost it.

Arado then started work on a Zerstörer with fixed armament. Project E 651 of 1937 envisaged a streamlined four-seat aircraft, with two Daimler-Benz engines buried in the wing/fuselage blending, driving tractor propellors on the wings through a gearbox and shafts. This was rejected by the RLM as too complicated, but it must have left a mark. When in 1938 work was started on a successor for the Bf 110, Arado was given the opportunity to compete with Messerschmitt, and encouraged to be radically innovative.

Meanwhile, the Bf 110 did become the standard twin-engined fighter of the Luftwaffe. The Zerstörer were soon regarded as the elite of the German air force, perhaps because they embodied the agressive spirit of the new German regime better than the short-ranged Bf 109, which primarily was a defensive fighter. Accordingly, the aircraft was given much publicity by the Nazi propaganda ministry, and the sleek Bf 110 became the inspiration for a series of foreign designs. After a decade in which twin-engined fighters had almost invariably been disappointments, the category suddenly became fashionable again.

To be fair to the air forces and design teams, both the advance of aeronautical technology and the tactical thinking of the time favoured the twin-engined concept. The single-engined fighter was evolving towards the combination of the smallest and sleekest possible airframe with the largest possible engine. This combination resulted in fast, agile fighters with a minimal range, such as the Bf 109 and Spitfire. These were highly optimized for air defense operations (directed by radar, which was secretly being developed in several countries simultaneously) against the new fast bombers, but handicapped in supporting offensive operations. This created a niche for a larger aircraft, and most assumed that it would have to be twin-engined to achieve anything like the performance of the smaller type. Advances in technology eliminated the numerous struts and wires that had handicapped the performance of older designs, and made it possible to design large fighters that were both aerodynamically efficient and esthetically attractive.

It remained to be seen how the new twin-engined fighters would perform in actual combat. At the outbreak of the Second World War, the Bf 110 was the equal or the better in performance of most fighters in service; only the Bf 109 and Spitfire were clearly superior. Over Poland, Norway, and France, against weak opposition in the air, the Bf 110 proved quite effective on offensive missions, including interception, air superiority and ground attack. But even over Poland, where the opposition consisted only of the antiquated PZL P.11 fighter, crews flying escort missions for bombers felt vulnerable, because they were forced to await being attacked. It would soon get worse.

Jachtkruiser

On 16 March 1937 an unusual shape made its first flight in the Dutch skies: The Fokker G.1, a twin-engined, twin-tailboom multi-role aircraft. The two-seat G.1 was classified by the Dutch as a “jachtkruiser” (cruiser pursuit) but could also serve as reconnaissance aircraft and light bomber, having a small internal bomb bay. The prototype had French engines, 690 hp Hispano-Suiza 14AB radials, probably because Fokker was hoping to get orders from France: There was no official Dutch requirement for the G.1, which was a private venture. The unreliable 14AB engines were later replaced by American 825 hp R-1535-SB4-G Twin Wasp Junior radials, and when the Dutch government placed an order, it chose the 830 hp Bristol Mercury VIII engine, which already powered other Dutch aircraft.

The G.1 was a clean cantilever monoplane, but under its skin the structure largely followed the traditional Fokker pattern: A wooden wing, with plywood skinning, and a fuselage that combined welded steel tube and wood. Only the tailbooms were light alloy semi-monococque structures. The short fuselage ended in a “Fokker conical turret”, a grand name for a cone that rotated around its horizontal axis, with a slot for a machine gun. The performance was not particularly exciting by the standards of the time, and the French decided that the G.1 wasn't better than their own Potez 63. However, it was slightly faster than the Fokker D.XXI, a single-engined aircraft with fixed landing gear, which was the Dutch standard fighter of the period. Fokker found a number of export customers for the two-seat G.1B, while the Dutch military ordered the larger, three-seat G.1A. (But most would be delivered with just two seats installed.)

The armament varied with the customer. The original proposal was to have two 23-mm Madsen cannon in the nose, plus two 7.9-mm machine guns, also from Madsen, and one similar weapon in the turret. A proposal to install 20-mm Hispano cannon had to be abandoned because of an excessive shift of the center of gravity. The Dutch G.1A had eight 7.9-mm FN-Browning M.36 guns in the nose instead, and another one in the tail cone. In May 1940 only 23 were available for service, and one more G.1A and three G.1Bs were rapidly added. The latter, built for export, received an interim armament of four FN-Browning guns in the nose.

After brief and unequal combat, the Luftwaffe overwhelmed the opposition. Production of the G.1 continued for the Luftwaffe, which used it as advanced trainer. This brought the total number of G.1s produced to 62.

German bombs destroyed the prototype of the Fokker D.XXIII, an ingenious twin-engined, single-seat fighter. The D.XXIII represented an attempt to achieve high performance by using two engines on a relatively small airframe: It featured two slender tail booms, and a short fuselage with an engine in the nose and at the rear. The prototype had relatively low-powered, air-cooled in-line engines, 540 hp Walter Sagitta I-SR inverted V-12s. First flown on 30 May 1939, it suffered from the predictable problems with the cooling of the rear engine, and only short flights were accomplished. Armament was to be two 13.2-mm and two 7.9-mm machine guns.

In late 1940 the Soviet design team of Aleksandr Moskalyev flew an aircraft that was clearly inspired by the D.XXIII, the SAM-13. It was smaller and lighter, with a loaded weight of only 1183 kg against 2950 kg for the D.XXIII, and powered by 236 hp MV-6 (licensed Renault) engines. Speeds up to 560 km/h are reported to have been achieved during the brief test programme, before this prototype too was destroyed during the German attack.

Polish hopes

In the middle of the 1930s, future planning for the Polish air force included the concept of a twin-engined multi-role aircraft, to serve as interceptor fighter, attack aircraft, and dive bomber. A mixed armament of cannon and machine guns was specified, while the recommended powerplant was the 8-cylinder, inverted, air-cooled Foka, which was expected to deliver around 600 hp. By 1936 the interceptor fighter role had been dropped, and the requirement refined to primarily one for a dive bomber and attack aircraft, with a secondary fighter role.

The P.Z.L. P.38 Wilk was structurally similar to the P.37 Los bomber, but smaller. It was intended to have two 20-mm Wz 38 cannon and two 7.7-mm machine guns in the nose, while a pair of 7.7-mm flexible guns was installed to defend the rear. Maximum speed was expected to be 520 km/h, but development of the Foka engine ran into severe difficulties, and by the end of 1937 it still was not ready for flight. In May 1938 the second prototype flew with American Ranger SGV-770B engines instead. These delivered only 450 hp, so could be regarded only as a stopgap. The first prototype made its first flight in January 1939 with improved Foka II engines. However, these too were not yet ready for production.

An alternative engine had been identified in the French Gnome-Rhône 14M Mars, a small-diameter radial. This was selected for the P.48 Lampart, a substantially redesigned aircraft, expected to be built in a version with two Wz 38 cannon and four Wz 36 machine guns in the nose, and one with eight Wz 36 machine guns. Top speed would was predicted to be 560 km/h. In September 1939, the prototypes were still being assembled, and both are believed to have fallen into German hands.

French rearmament

On 31 October 1934, the French air ministry issued a specification for a light multi-seat fighter. One of the roles envisaged for this aircraft was that of a flying three-seat command aircraft, flying in combat with an unit of single-seat fighters, carrying the latter's commander. More conventional roles were that of a two-seat escort fighter, and a two-seat nightfighter. The aircraft was to weigh less than 3000 kg (later increased to 3500 kg) and have a maximum speed of 400 km/h (soon raised to 450 km/h) at 4000 m. The fixed armament was specified as two 20-mm cannon.

France maintained a substantial number of small aircraft manufacturers, and a number of replies were received: Offers included the Breguet 690, Hanriot H.220, Loire-Nieuport 20, Potez 63, and Romano R.110. Potez offered the 63 with alternative Hispano-Suiza or Gnome-Rhône engines, the corresponding aircraft becoming the models 630 and 631, respectively. The Loire-Nieuport design never materialized, but prototypes of all others were built.

The Breguet 690 was a neat mid-wing monoplane with a dolphin-shaped fuselage and twin tail fins. With 680 hp Hispano-Suiza 14Ab radials, it reached a speed of 490 km/h at 4000 m. Armament consisted of two fixed 20-mm cannon and a 7.5-mm machine gun in the rear cockpit. The aircraft looked promising, but it made its first flight in March 1938, when the Potez 63 had already been selected to fill the fighter requirement. The 690 was further developed as attack aircraft.

The Hanriot H.220 looked unusual but sleek, with a short fuselage and, in its initial form, long nacelles for the Renault 12Roi in-line air-cooled engines, so that the propellers were well ahead of the nose. With these 450 hp engines the design was underpowered, so 680 hp Gnome-Rhône 14M radials were installed, but serious handling problems (and major structural damage following a forced landing) indicated a need for major redesign. For the time being, it was out of the running.

The Romano R-110 was remarkably awkard. Conservative in structure, with wooden wings and a steel-tube fuselage, it featured a second cockpit, stacked above and behind the normal position of the pilot. This was intended for use as aerial command post, the commander thus being provided with an excellent outside view. Although only powered by 450 hp Renault 12Ro in-line engines, the R-110 was credited with a top speed of 470 km/h, but like the Breguet 690 it flew in March 1938, too late to compete with the Potez 63. Development of this design, which appears to have had little enough potential, was discontinued.

The Potez 630 made its first flight in April 1936, powered by 580 hp Hispano-Suiza 14Hb engines. It was a very elegant, streamlined aircraft: A fuselage of oval cross-section, a low-set wing with straight taper outboard of the engines, close-cowled radial engines, and a tailplane with twin fins. During flight tests a tailplane with dihedral was installed to improved handling, and the engines were replaced by more powerful 14Ab of the same manufacturer. The Potez 630 reached 460 km/h at 5000 m. The second prototype was completed as a 631, with Gnome-Rhône 14M Mars engines, whose smaller diameter resulted in more streamlined engine cowlings, compensating for their lower power, 660 hp at rated altitude against 725 hp for the 14Ab.

The Potez 631 was selected and ordered into production, but because of a shortage of 14M engines, a larger number of Potez 630s were also ordered. The first production aircraft, a 630, was accepted in May 1938. The basic airframe proved robust, easy to fly, and adaptable. Orders also followed for derivatives, such as the 633 light bomber, the 637 observation aircraft, and the 63.11 reconnaissance aircraft. The observer in the 637 had to be content with a small ventral gondola with windows, while the 63.11 had a redesigned nose with large transparancies.

As usual, the production aircraft were slower than the prototypes. Measured top speed at 4,000 m was 448 km/h for the 630 and 437 km/h for the 631. On the other hand, the lighter 631 reached this altitude in 5 minutes and 6 seconds, while the 630 needed 7 minutes. For their size, these aircraft were too underpowered to show excellent performance; they fell short not only international standards, but also of the requirements of the current French re-armament plan, the Plan V. But nothing better was available, so relatively large orders were placed for the 631. It was decided to withdraw the 630 from first-line service, because of its unreliable engines.

Production remained disappointingly slow, hampered by shortages of engines, propellers, and guns. On 1 January 1939 the air force should have had a total of 201 model 630 and 631 fighters in service; it actually had 45 and 27, respectively. In the factories, 74 airframes were still awaiting engines, while many of the aircraft delivered had temporary two-bladed wooden propellers, instead of the metal three-bladed service unit. The planned armament of two 20-mm cannon was not available for the first 630s, so they were armed with four 7.5-mm machine guns instead. Many 631s had just one 20-mm cannon fitted, the other being temporarily replaced by a 7.5-mm gun.

At the outbreak of war the deliveries totalled 206 Potez 631s and 85 Potez 630s, but many had not yet reached combat units. In early 1940, the idea of using the aircraft as a flying command post was abandoned, and the last 630s were withdrawn from operational service. To improve the firepower of the 631s, the second 20-mm cannon would be installed during overhaul. It was also intended that four additional machine guns would be added under the wings, but only a handful of aircraft were so modified. Six night fighter groups (GCN, Groupe de Chasse de Nuit) were equipped with Potez 631s in May 1940; the type was no longer present in day fighter units.

Wisely, a programme to find a successor to the Potez 630 had been formally launched in 1937, before the aircraft entered service. The new A22 requirement specified a top speed of 550 km/h and heavy armament; the aircraft also had to have an autonomy of three hours at an elevated cruising speed, 90% of the maximum speed. The crew, for the moment, remained at three. Because of fears of a shortage of light alloys during wartime, the use of “non-strategic” materials such as wood and steel was recommended.

Further development of the 630 series produced the Potez 670, initially intended as a three-seat long-range fighter, but modified to have a crew of two before the first flight. It was a bit smaller than the 63, and the wing had an elliptical instead of a straight-tapered plan. The 670 was powered by 700 hp 14M radials, but the prototype was subsequently modified to represent the 671 with 800 hp 14AB 12/13 engines. Having made its first flight in March 1939, the type was still undergoing tests at the time of the French defeat, although work on a small production batch was already ongoing. With a speed of 500 km/h at 6000 m, the 671 did not meet the A22 requirement.

Hanriot offered the latest developments of the H.220. At first, the wing and engines were retained, and combined with a new fuselage and tail, this becoming the H.220-2. It made its first flight in March 1939 and showed a decent level speed performance, with a top speed of 532 km/h at 5,000 m. However, aerodynamic problems prompted further redesign, including the replacement of the wing. Because Hanriot had been absorbed in the SNCA du Centre, the aircraft was renamed the Centre NC.600. The first flight was made on 15 May 1940, after the German attack, precluding any possibility of series production. With two 710 hp 14M0/01 engines, the top speed was raised to 542 km/h. Armament was two 20-mm HS.404 cannon and two 7.5-mm MAC34 machine guns in the nose, and a flexible 20-mm cannon for the defense of the rear. The latter was stowed in a slot in the rear decking when not in use.

The most radically innovative of the A22 fighters was known at first as the Lioré et Olivier LeO 50. But before the prototypes were ordered Lioré et Olivier was absorbed in the SNCASE, the Societé Nationale de Constructions Aéronautiques du Sud-Est, also simply known as Sud-Est, and the aircraft became the SE.100.

The SE.100 was designed around a wooden wing, of limited area but with extensive flaps to keep the landing speed down. Instead of normal ailerons, a large part of the wingtip was made movable, this allowing more room for flaps on the trailing edge of the wing. The fuselage was constructed from steel tube and wood, was well streamlined, and unusually short; it ended in a large tailplane with twin fins. The landing gear consisted of a large retractable nosewheel and smaller wheels retracting into the lower part of the tail fins. Armament initially included two fixed 20-mm HS.404 cannon, and a single HS.404 on a hydraulically-powered mount to defend the rear; the latter could be neatly stowed in the rear decking of the fuselage when not in use.

The SE.100 looked futuristic, but below the sleek skin its wood and steel structure was heavy; the prototype weighed 5,732 kg empty and 7,679 kg loaded. With two 900 hp Gnome-Rhône 14N-0 engines, the SE.100 needed 8 minutes and 20 seconds to climb to 4,000 m; its top speed of 547 km/h at 4,400 m was well below the 600 km/h expectation of its designers. Hope was not abandoned; the design team considered production versions of the SE.100 with a lighter structure, more powerful engines, and much more powerful armament: One three-seat attack-bomber version was to have six forward-firing HS.404 cannon, a single HS.404 and two 7.5-mm machine guns in a ventral gondola, and a HS.404 at the tail. The defeat of May 1940 meant that all such plans were to remain dreams.

The Bf 110 in the Battle of Britain

Events were now leading to a confrontation between the Luftwaffe and the RAF, in which the Bf 110 would suffer heavily. Consequently, sharp criticism of the aircraft, and by extension the entire class of twin-engined fighters followed, well beyond what is justified by the actual events.

It is as well to start by looking at the performance parameters. The Bf 110C-4, a common model at the time, was powered by two 1,100 hp Daimler-Benz DB 601A engines, and had a normal loaded weight of 6,940 kg; so its power loading was 3.15 kg/hp. The Hurricane Mk.I weighed 2,924 kg loaded and was powered by a 1,030 hp Merlin Mk.III engine, giving a better power loading, of 2.84 kg/hp. The Spitfire Mk.I, powered by the same engine, was lighter because of its more advanced structure, and the normal loaded weight of 2,812 kg resulted in a power loading of only 2.73 kg/hp.

The Bf 110C-4 was claimed to be capable of a maximum level speed of 562 km/h at 7,000 m altitude, and 473 km/h at sea level. British test pilots, flying a captured C-5 reconnaissance version at lower weights, measured only 547 km/h at 6,700 m, so perhaps the German figures were too optimistic, but such variations between individual aircraft were common and the test aircraft had been assembled from two wrecks. This was still considerably faster than the 508 km/h at 5,400 m and 409 km/h at sea level of the Hurricane Mk.I, and competitive with the 557 km/h at 4,570 m of the Spitfire Mk.IA. The German fighter also performed reasonably well in the climb, with an initial rate of climb of 670 m/min, somewhat inferior to the 770 and 740 m/min attributed to the Hurricane and Spitfire. Nevertheless it could climb to 5,500 m in 7 minutes, while the Spitfire Mk.IA needed 6.85 minutes to 4,570 m (and the Hurricane 6.3 minutes).

During the battle of Britain, 100-octane fuel was used by RAF fighters. By allowing the Merlin to run at higher pressures and generate 1,200 hp, this considerably improved their performance, especially in climb. That came as an unpleasant surprise for the Luftwaffe. Nevertheless, in terms of performance the Bf 110 appeared competitive enough. Its weakness in combat against the RAF fighters was its manoeuverability, which was good for an aircraft of its size, but no match for the opposition it faced. The wing loading of the C-4 model, 181 kg/m², was considerably higher than the 123 kg/m² of the Hurricane and the 125 kg/m² of the Spitfire, and this resulted in a larger turning circle. The handling of the Bf 110 was quite good at normal speeds, with effective controls and generally pleasant flying characteristics. However, the controls heavied up considerably at higher speeds, and the ailerons were almost immovable in fast dives. But this, it must be said, was a weakness of all fighters at the time, German and British alike.

Overall, the characteristics of the Bf 110 implied that the Zerstörer were at a serious disadvantage whenever they were forced into a defensive position. Their speed and firepower served them well when they made hit-and-run attacks, exploiting their good climb and dive characteristics, while declining to engage in turning combat. Most fighter-versus-fighter combat during battle was of short duration anyway, typically less than half a minute; long dogfights were very rare because they left those who engaged in them in a very vulnerable position, low and slow, and too preoccupied with their target to watch their rear.

When flying Freie Jagd fighter sweeps at high altitude (above 6,700 m) over Southern England, the Zerstörer units demonstrated that the Bf 110 was indeed a dangerous opponent, and could be more than a match for the Hurricane. Unfortunately for the Germans, this was not the pattern of the battle to come. Fighter Command soon decided to avoid contact with the fighter sweeps, and concentrate its attacks on the bomber formations. Then, flying at low and medium altitude, and robbed of the advantage of surprise by radar and the advanced command-and-control system of Fighter Command, the German crews often found themselves at a serious tactical disadvantage. This, of course, also handicapped the Bf 109, but the nimbler single-engined fighter could extract itself from an unfavourable situation with much more ease. The normal reaction of Bf 110 crews was to seek safety in a Lufbery circle, a defensive formation in which fighters fly a ring pattern, each covering the rear of the one next ahead in the formation. While an effective tactic, this could only be maintained for as long as the fuel reserve allowed, and the RAF fighters waited above for an opportunity to strike.

Indicative of how just badly things could go wrong was an attack on Croydon on 15 August 1940. This attack was executed by bomb-carrying Bf 110s and Bf 109s of Erprobungsgruppe 210. Serious damage was done to airfield, but while still in its attack run at low level the Germans were bounced by a superior number of Hurricanes. They found temporary protection in a defensive circle, but finally had to make a break for home; the RAF fighters immediately fell onto their targets, shooting down seven out of the twenty-two strong enemy force.

The Bf 110 also had to take on the role of escort fighter for the bombers, because the fuel capacity of the Bf 109 allowed it only a short stay over England. But the German escort tactics were flawed. (Arguably, they were not much more flawed than some other attempts to provide bomber escort, such as the early efforts of the USAAF.) Too many fighters were ordered to fly close escort, in formation with the bombers; in that position they could do little to defend the bombers and became vulnerable targets themselves. The error was made worse in early September: Against the strong opinion of the fighter leaders, they were ordered to stay closer to the bombers, and even to break off combat with enemy fighters if the bombers were threatened. The Bf 110 turned out to be unsuitable for close escort missions, and losses were heavy.

That said, the often-repeated story that the Bf 110s themselves had to be escorted by Bf 109s is a myth. Tactically, this would have made no sense. The only Bf 110s that were really escorted by single-engined fighters were the fighter-bombers of Erprobungsgruppe 210, which were otherwise too vulnerable with their combat loads, or when making their attacks at low level. Perhaps this, or maybe the complex organisation of the escort fighters around a bomber formation (including formation flying distant cover, close cover, and top cover) gave raise to the story.

Anyway, the failure of the Bf 110 as an escort fighter was just one more nail in the coffin of a doomed operation, but as they had been a much-propagandized part of the German air force, it became symbolic. Its importance should not be exaggerated, for at the height of the battle, Bf 110s were less than 12% of the Luftwaffe's available force, and only just over a quarter of the fighter force. There were many more important factors that doomed the German attempt to subdue Britain to failure: It had seriously underestimated RAF strength, and ignored the importance of the radar and command-and-control network. The available bomber strength was insufficient, and in turn the fighter strength fell short of the requirement to provide escorts in a 2:1 ratio. Aircraft production and crew training lagged behind the RAF. Intelligence and target identification were very poor, so that many bombs were wasted on targets of little importance, and enemy losses strongly overestimated. The range of the Bf 109E was too short, the use of drop tanks was neglected, and the available bombers had small bomb loads and weak defensive armament. Last but not least, the operation itself was handicapped by two conflicting goals, preparing for an invasion by destroying the RAF and forcing Britain to come to a negotiated peace by bombing, and neither was a realistic war plan.

The catalogue of failure should not obscure that the Bf 110 had also found a role in which it was very effective, that of a fighter-bomber. This was especially so in the hands of Erprobungsgruppe 210, a unit that specialized in precision attacks on targets such as airfields, radar stations, and shipping. At the beginning of the battle it operated a number of Bf 110C-6 with a powerful MK 101 30-mm cannon in a fairing under the fuselage, but operational and technical problems lead to the removal of this weapon. The occasional heavy losses demonstrated the need for air superiority on such operations, but a series of successful attacks also confirmed the effectiveness of the aircraft in this role. The large Messerschmitt fighter still had a long and generally successful career ahead of it.

RAF

As we have seen above, none of the British twin-engined fighter projects during the interbellum produced an useful combat type. Instead, single-engined two-seat fighters were in service, the latest biplane representative being the Hawker Demon, a development of the Hart light Bomber. Its direct technical successor was arguably the Boulton-Paul Defiant, but there also appeared to be an operational requirement for a twin-engined, long-range fighter. It so happened that a convenient solution seemed to be around in the form of the Bristol Blenheim light bomber, an aircraft with a fairly high performance. Blenheim Mk.I bombers were already being replaced by later versions in Bomber Command, and after some debate a very simple conversion of these aircraft allowed them to be used as heavy fighters. The changes consisted of four .303 machine guns in a ventral gun pack, light armour plate for the pilot, and a gunsight (or more precisely two, a reflector sight and a ring-and-bead sight as backup). This version was known as the Blenheim Mk.IF. It also retained the simple (and rather ineffective) Bristol dorsal turret with a single Vickers Class K gun, and a fixed gun in the left wing.

Introduced in late 1938, this amounted to a conversion of a faintly obsolescent light bomber into an even more obsolescent heavy fighter, underpowered and undergunned. With 840 hp Mercury VIII engines and a top speed of 423 km/h at 3,050 m, the Mk.IF was no match for the Bf 110, and combat against enemy single-seat fighters clearly had to be avoided. As a day fighter, the Blenheim was best used only as a trainer, although some went to Coastal Command. (In vain, this Command pressed for the replacement of the .303 guns by two 20-mm cannon.) It was the best available candidate for conversion into a radar-equipped nightfighter, however, despite the disadvantage of reflections in the complex cockpit glazing.

The installation of radar was done in great haste. The first demonstration of the prototype AI radar set (installed in a Fairy Battle) was done in June 1939. It was quickly decided to equip 30 Blenheims, and 15 sets were delivered at the beginning of the war, to No 25 Squadron. This did not have much impact on the war, because the ground control, while good enough to intercept formations in daylight, was quite incapable of putting a nightfighter within radar range of an enemy target. Not until 1941 was an effective “Ground Controlled Interception” (GCI) radar available.

Meanwhile, work proceeded on a number of other designs, in fact quite a large number, and designed to a series of different specifications: F.37/35, F.18/36, F.9/37, F.11/37, F.18/37, F.22/39, F.18/40, and others. A few designs were built, and the Westland Whirlwind even briefly entered service. But the outcome of all this work was that the RAF fought the war with two twin-engined fighters, the Beaufighter and the Mosquito, which were fortunate by-products of other programmes.

There was no single cause for this state of affairs, but several important ones can be identified. Many of the first generation of twin-engined designs were intentionally kept small, to stay close to the weight and size of single-engined fighters. But this not only limited their usefulness and development potential, it also tied them to lighter engines of lower power, while wartime efficiency dictated that only a handful of the most powerful engines were to be mass-produced. Other designs were prepared to carry powerful cannon armament in flexible mounts, up to quadruple 20-mm cannon or 40-mm cannon. This resulted in aircraft that were far too big and heavy, and was tactical nonsense. Finally, first the need for high production of defensive fighters and later the excellence of the Mosquito blocked the way for a number of designs with high potential, of which the production could not be justified on grounds of industrial policy.

Sad Whirlwind

The unfortunate story of the Westland Whirlwind illustrates the point. It originated from specification F.37/35 for a fighter armed with four 20-mm or 23-mm cannon, a weapon that had been selected to arm the next generation of RAF fighters. (The Hispano had been selected, but apparently the choice between the 20-mm HS.404 or a 23-mm development of this weapon had not yet been made.) The specification left open whether the aircraft would have one or two engines, but design teams apparently felt that they needed 1600 to 1800 hp, an amount of power that could not be delivered by a single one of the engines in production at that time. The most promising of the single-engined designs was judged to be the Boulton-Paul P.88, offered in versions with a Bristol Hercules or Rolls-Royce Vulture engine, neither of which was immediately available. A (rather more elegant) offering by Bristol, the Type 153 design, also featured a Hercules radial, but Bristol also created the Type 153A, a small fighter powered by two Bristol Aquila radials. The 153A had some features in common with the Grumman XF5F-1 Skyrocket, including a fuselage nose that did not extend beyond the leading edge of the wing, radial engines in short stubby nacelles spaced closely together, and two small tailfins. The four cannon were in the bottom of the fuselage. It looked neat, but with hindsight the aerodynamics of it are rather suspect, and perhaps it is fortunate that the Type 153A was not built.

At Westland, W.E.W. Petter solved the problem by designing a small, trim fighter with two Rolls-Royce Peregrine engines, rated for 885 hp at 4,600 m. It was a very highly streamlined design, not much bigger than a Hurricane or a Spitfire. The Westland P 9 design also featured a magnesium alloy monocoque fuselage, relatively small wing with large Fowler flaps, integral fuel tankage, radiators buried in the wing roots, and an all-round vision cockpit. The four 20-mm cannon Hispano were closely grouped together in the nose. It really was an innovative and attractive design, but in September 1936 the RAF also chose Westland because it had a less busy schedule than other manufacturers, its main other task so far being the development and production of the Lysander close-support aircraft. The order for the P 9 was confirmed in February of the following year.

There was a risk in choosing a small and relatively inexperienced company without large production facilities. The prototype flew in October 1938, but production was not expected before June 1940; the schedule continued to slip and the first operational squadron was only equipped in December 1940. Even in the first production aircraft many modifications were made, which confirmed the RAF's fears that Westland was not capable of the mass production of a modern fighter. The company defended itself by arguing that the Whirlwind's engine and handling faults were not worse than those encountered by most fighter prototypes, which was probably true. But Westland was too small to solve them quickly, the RAF was losing confidence, and Rolls-Royce gave the Peregrine a low priority. In August 1939 staff officers were already considering the replacement of the F.37/35 by a new design. There was nothing wrong with the performance of the aircraft, which at low altitude was faster than a Hurricane or Spitfire, and highly manoeuverable. Its misfortune was that because of all the delay, it entered service at a time when other single and twin-engined fighters were already being adapted to the same missions, and in many ways showing themselves more suitable.

And the choice for the Peregrine engine had been a fundamental error. About 20% less powerful than the Merlin, this engine was used by no other aircraft, and it was soon obvious that the war required Rolls-Royce to concentrate its effort on a few much-needed engines, each the most powerful in their class. Continued Peregrine production was only possible by building fewer Merlins, and because of reduced efficiency the exchange would be at a rate of perhaps two to one, meaning that one Whirlwind would cost the engines for four Spitfires or Hurricanes. Or, alternatively, by postponing the production of the much-needed Vulture or Griffon, which was also out of the question. Therefore, when Rolls-Royce stated that these were the options from which a choice could be made, the fate of the Whirlwind was sealed. To be fair to Westland, Bristol's choice for the Aquila radial, another small engine, was not any better; and Supermarine suggested to use the doomed Rolls-Royce Goshawk or even the Hispano-Suiza 12Y in its Type 313. All twin-engined offerings to F.37/35 suffered from the same problem.

Anyway, in May 1940, months before the fighter even entered service, it was decided that Peregrine production would end in December; therefore production of the Whirlwind would have to be stopped after only 114 had been delivered. The type continued in service until December 1943, mainly in the fighter-bomber role with a 500 lb bomb under each wing. With this still relatively modest load the Whirlwind handled well, and the cannon were quite useful for strafing.

The installation of the Merlin might have salvaged the concept, but at the time Westland rejected this; the airframe was too small to take a bigger engine. (Later, Petter admitted that it would have been better to design the aircraft around two Merlins from the start.) Westland proposed to cure the main faults of the Whirlwind in a Mk.II version, which would have a larger internal fuel capacity, 120 instead of 60 rounds per cannon, and further developed Peregrine engines – but Rolls-Royce poured scorn on that idea. Another alternative suggested by Westland was the installation of American radial engines, perhaps the R-1830 or R-1820. The RAF had already lost interest in the Whirlwind, so these were dead ends.

Size matters

The twin-engined fighter that would do most to replace the Whirlwind was born out of a very different project. In October 1938, the first flight was recorded of the Beaufort, a twin-engined torpedo-bomber that would have a useful but troubled service life. Within the next few days, Bristol proposed the Type 156, a fairly straightforward development that would retain the wings and tail of the Beaufort, with a new front fuselage and more powerful Hercules radials to replace the rather unsatisfactory Taurus engines of the Beaufort. Four cannon would be installed in the bottom of the new front fuselage. (After the first 50 aircraft, six .303 wing guns were added.) Bristol predicted a top speed of 580 km/h at 4,600 m. And crucially, it indicated that this aircraft could be available early in 1940, even calculating that it would be able to produce 100 of them before the end of 1939. The idea was accepted, and specification F.17/39 was written around Bristol's design.

The story is told that W.E.W. Petter was invited to have a look at the mock-up of the competing design, and commented that it was “far too large, especially the fuselage.” To which Bristol's designer Frise replied that in two years time, such size would be necessary – and he was right. Size saved the Beaufighter, as it would be called, when the Whirlwind was cancelled. Because of its size, it had much more development potential, and unlike the Whirlwind, could things no single-engined fighter was capable of. The pugnaciously ugly Beaufighter was an aircraft the RAF refused to do without, even when concerns about delivery and performance reached the alarm level.

It soon became clear that the Hercules VISM engine, for which the original performance estimates had been made, would not be ready for some time, and that the first production aircraft would have the less powerful Hercules II or III. The optimism of early production schedules could not be maintained either; the first prototype flew in July 1939 and the first production examples reached operation squadron in September 1940. Three squadrons were fully equipped with Beaufighters before the first Whirlwind squadron became operational, but this had been achieved by skipping final testing stages, and as consequence the level of serviceability of these aircraft turned out to be quite low. Especially the cannon caused a lot of problems, because the Hispano demanded a very stiff mounting. Early Beaufighters carried their 240 rounds per cannon in individual drums, and the hapless observer had to exchange these heavy objects, groping around in a cluttered and dark fuselage. After much delay, a belt feed mechanism was developed and installed.

Tests by the A&AEE of the first four prototypes, with different engines, revealed top speeds between 539 km/h and 483 km/h, considerably inferior to the Whirlwind and a cause for serious concern. However, the Beaufighter was ideally suited as a nightfighter, because there was room in the fuselage for radar and a radar operator, and was also eagerly awaited by Coastal Command. The Mk.IC version for Coastal Command had extra fuel tankage instead of the 7.7-mm wing guns, giving it a range of 2900 km.

But performance really had to be improved. With Hercules III engines, the top speed of a production Mk.IF was 520 km/h at 4,400 m; with Hercules X or XI engines this was raised to 539 km/h at 4,700 m. Concern about Hercules engine production lead to the Beaufighter Mk.IIF version powered by Merlin XX engines, which during tests gave this model a better high altitude performance, 534 km/h being reached at 9,150 m. But these good results were not reproduced on production aircraft, and in addition the different shape of the engine nacelles created aerodynamic problems. The 1,670 hp Hercules VI or XVI, when finally available, was installed in the Beaufighter Mk.VI, but the performance improvement with the new engines was disappointing.

More hopes were raised by the prospect of installing Griffon engines, or entirely redesigning the fuselage to reduce drag. Because of wartime production thes projects were not pursued, and the introduction of the superior nightfighter Mosquitoes in mid-1942 removed much of the need for a faster Beaufighter. From then on the Beaufighter was gradually phased out as a nightfighter, although Fighter Command did not release the last ones until June 1944. Four USAAF squadrons, operating in the Mediterranean, were also equipped with nightfighter “Beaus”.

The Beaufighter then primarily evolved into a highly successful strike fighter. It had generally good handling characteristics, but early “Beaus” suffered from longitudinal instability, and the Mk.VI would introduce a tailplane with strong dihedral to improve stability at the cost of some loss in manoeuvrability. The heavier TF Mk.X also needed a large addition to the dorsal fin. As a daylight fighter the aircraft was not a great success, as it was inferior in speed and manoeuverability to the Bf 110. But it was an immensely strong, rugged aircraft, capable of flying long patrol missions and delivering a heavy war load. It was an ideal aircraft for interdiction and anti-shipping missions.

Coastal Command took almost all of the Beaufighter Mk.X production. The Beaufighter was perfectly suitable to attack the German convoys or U-boats in the North Sea and the Bay of Biscay, with their heavily armed escorts. While some operated as fighters, others carried bombs or eight rockets under the outer wings. The TF models, also known as “Torbeau”, carried a torpedo, fitted ith a “Mono Air Tail” to stabilize it in the air and extend its range of allowable launching conditions. Thus the development of the aircraft had gone full circle: Originally derived from a torpedo-bomber, the Beaufighter had now itself become a very successful torpedo launcher.

F.9/37

One of the neatest of the twin-engined fighters was built to Specification F.9/37, which in turn was inspired on work by Gloster to earlier specifications for a two-seat fighter, armed with a dorsal turret. Originally this required a twin-engined fighter with a fixed nose armament of two 20-mm cannon, installed at the so-called ‘no-allowance angle’ at which projectile body lift compensated for gravity, as well as a retractable four-gun dorsal turret. But before Gloster's G.39 took the air in April 1939, the dorsal turret had been abandoned. Instead, three extra cannon were to be installed in the aft fuselage; because of their no-allowance trajectory they were aimed a few degrees upwards, firing over the head of the pilot. With the original 1,050 hp Taurus T-S(a) radials the G.39 demonstrated a speed of 579 km/h at 4,600 m, but these engines were troublesome and still about a year away from reaching maturity. With 900 hp Taurus T-S(a)3 engines the speed dropped to 534 km/h, and with 880 hp Peregrine in-line engines the second prototype attained 531 km/h.

The G.39 was a neat and manoeuverable aircraft, but it considered too small for development, and the late change in armament had left it with a center of gravity problem. For some time the delay in the development of the Beaufighter, and the downward revisions of its performance figures, appeared to create an opportunity for the G.39. However, the Taurus and Peregrine engines were non-starters. In the spring of 1940 Gloster started work, at a suggestion from the Ministry of Aircraft Production, on a development with two Merlin XX engines. Called the 'Reaper', this was to carry four 20-mm cannon and eight machineguns in its single-seat version, and four cannon in its two-seat version. At the end of 1940 it was judged to be the best night fighter design that could be expected. However, Gloster was already engaged in the development of jet fighters, and chief designer George Carter warned that he did not have sufficient design staff to develop both the Reaper and a jet fighter. Furthermore, it would take at least 18 months before the aircraft could enter service. With the Mosquito available to take on the night fighter role, the Reaper design never progressed beyond the drawing board.

Wooden Wonder

The best of the British wartime twin-engined fighters was the de Havilland Mosquito. While this originated as a fast unarmed bomber, its potential as a fighter was obvious. It was, in fact, hard to decide whether the Mosquito was most desirable as a reconnaissance aircraft, a bomber, or a fighter; an uncertainity that created some problems with the organisation of the early production runs.

This is not the place to repeat the history of the development of the Mosquito bomber. When the first order for fifty aircraft was placed on March 1940, this included only bomber and reconnaissance models; but the project was shelved a few months later when Lord Beaverbrook reorganized aircraft production to concentrate it on the most urgently needed types only. Work on the Mosquito, never really halted, was officially restarted in July; and in that month de Havilland also received instructions to complete one of the prototypes as a fighter, with four 20-mm Hispano cannon in the belly and four 7.7-mm Browning machine guns in the nose. A contract for fighters, to specification F.21/40, followed in November, also the month in which the first prototype made its first flight. At that time, there was still discussion about the final role of the type, and a long-range day fighter version to protect convoys against German bombers was considered.

The Mosquito was, famously, built from wood. In a way, the Mosquito preceded modern trends in aircraft design, where increasing use is made of composite materials, often sandwiched around a foam core. Wood is a natural composite, consisting of cellulose fiber in a lignin matrix. For the fuselage, de Havilland opted for a sandwich consisting of skins of birch plywood with a balsa filler, this structure being three times as a strong as a plywood panel without a balsa core. The fuselage halves where glued together over a concrete mould, each plywood skin consisting of three layers 1.5 to 2 mm thick, arranged with the grain of the layers of wood at 45 degrees to each other. The wings had wooden box spars (consisting of laminated spruce flanges and plywood webs), spruce stringers, and plywood skins.

Important advantages of such a structure were that it needed a relatively small amount of light alloys (although the required high-quality wood was not that easy to find) and that it allowed woodworking shops to take part in the war effort. Another important advantage of plywood was that it could be given a very smooth finish, reducing drag. Close-cowled Merlin engines, radiators buried in the wing roots, careful design of the propellor hubs, and great attention to the details of streamlining resulted in an excellent performance: The first prototype recorded a top speed of 624 km/ h at 6,700 m. For about two and a half years the Mosquito could be said to be the fastest aircraft in operational service, also thanks to new versions of the Merlin engine.

This level of performance, in combination with initial doubts about the concept of an unarmed bomber, resulted in a decision in the summer of 1940 to give the fighter version a higher priority than the bomber. Official vacillation about the role of the first Mosquito production aircraft made de Havilland's work more difficult. While there was much commonality between fighter and bomber models in the fuselage structure (of course a nosecone with gun mounts replaced the transparent nose cone of the bomber version), the fighter model had a different, stronger wing, to take the higher loads of combat manoeuvering. It also had an armoured glass windscreen. The Mk.II fighter version finally flew its operational mission at the end of April 1942 — about a month ahead of the first operational use of the bomber model.

These fighters were F Mk.II models, most of them operating as NF Mk.II nightfighters with AI Mk.IV or Mk.V radar. They were much faster than the Beaufighters they replaced, and had a higher service ceiling, better handling and greater manoeuverability. Armament consisted of four 20-mm cannon in the belly and and four 7.7-mm machine guns in the nose. The aircraft was soon sent on offensive missions as well, stripped of its radar and equipped with extra fuel tanks, that allowed it to range deep over Germany. Later this “Intruder” role passed to the FB.VI, which could carry 1,000 lb of bombs in an internal bomb bay.

The number of roles of the fighter Mosquito soon expanded. Nightfighter Mosquitoes escorted RAF bombers into Germany, taking an increasingly heavy toll of the German Nachtjagd. Conventional escort operations were accompanied by Flower intruder operations that targeted German fighter bases, while on Mahmoud missions the British nightfighters deliberately offered themselves as bait, in the expectation of turning the tables on any attacker. Ranger missions were flown over German-held territory in day and night, looking for targets of opportunity, and Instep patrols over the Bay of Biscay and the Western Approaches targeted German long-range fighters and other aircraft over the travelling routes of the German submarines. In the 2nd Tactical Air Force, fighter-bomber Mosquitoes replaced light and medium bombers, making pin-point raids on selected targets and providing air support for the invasion forces after D-Day. In Coastal Command, Mosquitos flew anti-shipping missions over the Atlantic ocean and the Norwegian coast. And in June 1944, two Mosquito nightfighter squadrons were diverted to the interception of V-1s at night.

To cope with a wide range of tasks and increasing demands on the aircraft, the Mosquito was repeatedly upgraded, but due to the high demand for the type, older models had to serve until worn out, or were modified to bring them up to the most recent standards. In the nightfighter line, the NF Mk.II with Merlin 21 engines and AI Mk.IV or Mk.V radar (466 completed) was followed, in January 1943, by the NF Mk.XII with AI Mk.VIII radar. The latter was a centimetric radar set, giving much better results than the older radars; a number of Mk.II models were converted to Mk.XIIs. As the new radar was in a radome on the nose, the 7.7-mm machine guns were removed. The similar NF.XVII was equipped with the AI Mk.X radar, alias SCR 720, a development of the British centrimetric radar produced in the USA. The NF.XIII of February 1944 was a new-built model that combined the AI Mk.VIII with the airframe of the FB.VI, which incorporated outer wing fuel tanks, and an internal bomb bay for intruder missions. The engine remained the Merlin 21 or 23, but fifty NF.XIII were equipped with nitrous oxide injection to boost their high-altitude performance for short periods — a speed gain of 75 km/h was measured at 6100 m.

The NF.XV, which turned out to be superfluous, was a highly specialised high-altitude model with extended wing tips, only four 7.7-mm Browning machine guns as armament, and Merlin engines with two-stage superchargers; only four Mk.IIs were converted to this standard, which boasted a service ceiling of 13,595 m. The NF.XIX of April 1944 had the airframe of the Mk.XIII, more powerful Merlin 25 engines, and could carry either AI MK.VIII or Mk.X in an “universal” nose radome. The final wartime nightfighter model, the NF.30, was a derivative of the NF.XIX with 70-series Merlin engines with two-stage superchargers; this gave a much better high-altitude performance with top speed raised from 595 km/h at 4270 m to 682 km/h at 8,075 m. It entered service in the spring of 1944.

The fighter-bomber models were an offshoot from the early success of the fighter models. The FB Mk.VI was essentially a derivative of the NF.II with the ability to carry bombs, both internally (in a small bomb bay aft of the cannon) and externally (under the wings). From October 1943 onwards, the aircraft could also carry eight rocket projectiles under the wings. The success was so great that ultimately, one third of the Mosquito production run consisted of fighter-bomber versions. This was helped because the fighter-bomber model was also built in Canada (the main production model being the Mk.26, with Packard Merlin engines) and in Australia as the Mk.40. For the anti-tank role, a small number of FB.XVIII ‘Tsetse’ were produced, armed with the powerful 57-mm Molins cannon. But ultimately the RAF had misgivings about the usefulness of this type and gave the aircraft to Coastal Command for anti-shipping missions.

The Mosquito was a great success as a nightfighter and a fighter-bomber, clearly superior to the Ju 88, Bf 110 or Me 410. But it was unwise for Mosquito crews to engage in combat with enemy single-engined day fighters, if they could avoid it. The Mosquito was fast but it could not match a Fw 190 or Bf 109 in manoeuverability or rate of climb. While realizing the maximum of the potential of the twin-engined type for the period, the Mosquito had sacrificed the ability to fly the missions of a single-engine fighter. That probably was an unavoidable choice, and the war potential of the Mosquito proved that it was a good one.

Failed Efforts

In the Beaufighter and Mosquito, the RAF had two of the most successful twin-engined fighters of the period. Curiously enough, both were a side product of bomber development, although before and during the war there were many other efforts to develop twin-engined fighters. Of the resulting designs some had a high potential but were abandoned because the types in service already met the operational need, while others were embarrasingly misguided.

Specification F.18/37 (actually issued in March 1938) called for a high-speed single-seat fighter, with a top speed not less than 400 mph at 15,000 ft, and an armament of twelve Browning machine guns or more. The Bristol, Gloster and Hawker submissions were single-engined, but Supermarine offered twin-engine designs. It argued that for this category of aircraft, a twin-engined type could actually be smaller than a single-engined type, and it offered a better forward view and (with airscrews rotating in opposite directions) better handling. The Type 324 and 325 both featured a wing of elliptical platform, just 20% larger than that of a Spitfire, and a slender fuselage with a short nose. Guns were installed in the outer wing panels. The Type 324 had its engines installed as tractor engines, while the 325 had pusher engines; in both case there was a choice between Merlin V-12s or Taurus radials. The Supermarine offerings lost out to the Hawker design (which became the Vulture-powered Tornado and the Sabre-powered Typhoon) because the Air Staff saw little advantage in the twin-engined layout.

But Supermarine's design was given a second chance, although not without doubts, when the Air Staff decided to switch to six-cannon armament. The resulting Supermarine Type 327 was a tractor design with six cannon installed very closely together in the wing roots, and a preference for the Merlin engine. A mock-up of the 327 was built, but there was a good deal of skepticism on the practicality of this design. Supermarine could not be expected to develop the 327 quickly, and there were already three twin-engined fighters under development. The decision was made not to put this aircraft in production, despite its promise.

Other efforts to develop a twin-engined fighter followed a rather more complex storyline, with different specifications reflecting that the ideas on this subject were still evolving. Specification F.6/39, issued in April of that year, called for a powerful “fixed gun fighter”, mounting four 20-mm or two 40-mm cannon. But F.22/39 of September imagined an aircraft with a 40-mm cannon in a nose turret, powered by two Griffon engines, and capable of at least 400 mph at 20,000 ft. The latter specification morphed into F.16/40 for a “fixed cannon gun fighter”, basically the F.22/39 modified to carry eight 20-mm cannon fixed to fire forward, with 200 rounds per gun. The final iteration of the specification was F.7/41, which converted the aircraft into a high-altitude fighter with a pressure cabin, reaching its top speed of at least 415 mph at 33,000 ft, powered by two Merlin 61 engines, and with six cannon with 120 rounds each.

The unfortunate recipient of this bewildering series of specifications was Vickers, which mounted a six-year effort that resulted in one completed prototype and very little flying. However, it at least partially had itself to blame. When F.6/39 was issued, the design team of Rex Pierson drew the RAF's attention to its Type 414, designed to carry a Vickers S 40-mm cannon in the nose, and a predictor fire control system operated by a gunner who sat next to the pilot. (A concept similar to the Bell FM-1 Airacuda.) The gunner would aim through a sight, and his input would be fed into a predictor unit, through flexible shafts and photo-electric units that would make corrections for the own speed and height. The predictor unit would aim the nose turret, which allowed for 45 degrees of elevation, 5 degrees of depression, and 20 degrees of aim to the side. It was this Vickers design that triggered the issue of F.22/39, while F.6/39 was cancelled.

In April 1940, following an inquiry on the possible installation of fixed guns, Vickers offered the Type 420, with eight 20-mm or two 40-mm cannon in its nose. The fuselage was redesigned to reduce the cross section, and the second crew member now set behind the pilot, neat the trailing edge of the wing. The elliptical wings were swept slightly forward, giving the design an unusual shape. The RAF issued F.16/40 to cover the Type 420, and ordered both the F.22/39 and F.16/40 designs to be built. Early in 1941, the RAF had apparently lost interest in both aircraft, which were rather too large and heavy to fill the twin-engined fighter role effectively. Vickers countered by offering a high-altitude version with Merlin engines and a pressurised fuselage, claiming that it would be easy to seal and pressurise the circular-cross section, stressed-skin fuselage. The RAF duly cancelled the F.22/39 aircraft, and at the end of the year issued F.7/41 to cover the new Type 432. This was to carry a pilot only, confined to a primitive pressure cabin, 5 m long and 1.4 m in diameter. This could be entered through a heavy circular door installed in the fuselage; but the transparent canopy, a round blister, could also be opened. The six cannon were to be installed in a ventral pack under the whale-shaped fuselage.

Nicknamed “Tin Mossie” because of its superficial similarity with the Mosquito, the aircraft finally made its first flight on 24 December 1942. Unfortunately, the type 432 was beset with aerodynamic and handling problems, engine cooling problems, and unsatisfactory performance. By the autumn of 1943, the 432 was due to be cancelled, but in a last confusing iteration, Vickers proposed to use the aircraft to test an installation of a 40-mm cannon with a predictor sight. It is possible that some work on this was done before the aircraft was finally scrapped, but it was not flown in this configuration.

The conversion of the Vickers project to a high-altitude fighter had been prompted by fears that German bombers would switch to operations at very high altitude. The development of pressure cabins and turbochargers made such operations technically feasible, justifying the development of a fighter to counter this treat, which in the end did not materialize (except for a small number of high-altitude reconnaissance aircraft). In the summer of 1941, Specifation F.4/40 for a single-seat high altitude fighter was released, stipulating a design service ceiling of no less than 45,000 ft. The bulk and weight of the pressurised cabin, and the reduced power of the suggested Merlin RM.6SM engines at high altitudes, made a twin-engined aircraft the most practical solution, but a single-engined design was also possible.

General Aircraft, the only manufacturer with practical experience in the design of pressure cabins, suggested the GAL.46, an elegant design with twin tail fins and large wing root fairings, blending into a slim fuselage. However, General Aircraft lacked any experience in fighter design. Westland, on the other hand, suggested an larger extrapolation of the Whirlwind with Griffon or Merlin engines and a long-span wing with a high aspect ratio. A single-engined offering by Hawker, based on a Typhoon with a Sabre engine and two-stage supercharging, was actually liked best; but Westland won the contract. The Westland F.4/40 was ordered as the Welkin, in Mk.I version as a single-seat day fighter and Mk.II as two-seat nightfighter, both with Merlin engines, with Griffon engines planned for a later version. When F.7/41 was issued, the Welkin design was modified to meet this specification as well.

The Welkin was the biggest single-seat fighter of its time. It looked good, with the angular contours of the wing and tail surfaces complemented by a streamlined fuselage and closely-cowled Merlin 61 engines. The pilot, seated well forward, enjoyed an excellent forward view. The first flight was made on 1 November 1942, after a creditably short development period. To avoid leakage problems with pushrods or cables moving through the pressure wall of the cabin, much of the systems were electrically controlled. This turned out to be something of a maintenance nightmare, while serious problems with the engines and propellers would be a recurrent theme during testing. The Welkin also needed some aerodynamic modifications need cure handling deficiencies. However, what really handicapped the Welkin was the relatively thick section of its wing, a 21% thickness/chord ratio at the root tapering down to 15% at the tip, which gave it a critical Mach number of 0.74, with severe pitch oscillation already at Mach 0.68. This was not acceptable for a good high-altitude fighter, but fortunately the operation needs could be met by other aircraft. Although 103 Welkins were completed, most were put in storage, and the Welkin never equipped a squadron.

Italy

The twin-engined fighter also attracted the attention of the authorities and manufacturers of Fascist Italy. They were in a poor position to take up the concept, because in the late 1930s the Italian aviation industry and air force were conservative, entrenched defenders of the agile but slow biplane fighter (they fielded the last of its kind, the Fiat C.R.42). Equally harmful was a lack of modern and powerful engines, which was only compensated by building German Daimler-Benz engines in license when it was already too late. Old-fashioned structures and low engine power were probably more detrimental for heavy fighters than for any other kind of aircraft. Besides, the available production capacity was small and poorly managed, and it was impossible to build a sufficient number of these aircraft to make their development worth while.

Much of this shows in the Fiat CR.25, flown in 1937 as a twin-engined long-range fighter. Roughly the size of a Mosquito and somewhat lighter, the CR.25 had a crew of three and was powered by two Fiat A 74 RC 38 radial engines of 840 hp. With these engines and a clean but conventional design, the CR.25 had a maximum speed of 460 km/h at 5,500 m, creditable but far from brilliant. The production CR.25bis version had a modest armament of just two fixed 12.7-mm Breda-SAFAT machine guns and one flexible gun of the same type in a dorsal turret. Just 10 served operationally, and in defiance of their apparent vulnerability they soldiered on until 1943 without operational losses, despite clashes with Hurricanes, Blenheims, Beaufighters and even P-38s.

A compact aircraft produced by IMAM took the other path in the development of twin-engined fighter. The Ro 57 was a bit smaller even than a Whirlwind, and powered by the same A 74 RC 38 radials as the CR.25. The Ro 57 was considerably slower than the Whirlwind, however, with a top speed of 500 km/h, and carried much weaker armament, just two 12.7-mm machine guns. A less advanced structural and aerodynamic design appears to have been the cause, although the Italian fighter took to the air a bit later, making its first flight in early 1939. Official doubts about the usefulness of this type of aircraft and its ability to survive combat with single-engined opponent took its toll, and the Ro 57 finally entered limited service as a fighter-bomber in early 1943. It is useful to compare this with the development time of the Whirlwind, which was judged much too long by the RAF; that of the Ro 57 took about two years more!

The Caproni firm, founded by Gianni Caproni, had made its reputation during the First World War as the manufacturer of large bombers. The typical Caproni aircraft of the immediate post-WWI period were sesquiplanes, but in the early 1920s drawings were made for a twin-engined monoplane fighter of all-metal construction, with a blunt-nosed fuselage, a cantilever wing, corrugated metal skinning and fixed landing gear. A quite radical design for the time, this Ca.65 was not built.

The firm had another opportunity in 1940 when Caproni built the elegantly streamlined Ca.331. This aircraft possesed a modern all-metal monocoque structure, highly streamlined lines, two 750 hp air-cooled in-line Delta RC.35 engines, and in its initial reconaissance version, a smooth transparent nose. In May 1942 the air force ordered the conversion of the second prototype in a nightfighter version, with a stepped cockpit and a solid nose, four 20-mm MG 151/20 cannon, two 12.7-mm Breda-SAFAT machine guns, and a flexible Breda-SAFAT in a dorsal and ventral defensive position. While the aircraft looked good, it was underpowered for a fighter, even after the installation of 800 hp engines or with the planned 850 hp engines. Top speed of the production version would have been 505 km/h at 5300 m, but Italy surrendered before production could be seriously contemplated.

SIAI-Marchetti contributed one of the more radical concepts. The S.M.91 had the same general configuration as the Lockheed P-38 Lightning, twin tails booms connected by a tailplane and a central nacelle for the crew. The S.M.91 was a rather bigger and heavier than the P-38, however, with a crew of two and about a third more wing area. As flown in March 1943 with 1,290 hp Daimler-Benz DB 605A-1 engines, the top speed was a modest 585 km/h at 7,000 m. In the hope of reducing drag and increasing performance, the team developed the S.M.92 in parallel; this had no central nacelle, and the crew was accommodated in a cockpit in the left-hand tail boom. Armament had to be relocated as well, and while the the S.M.91 had three MG 151/20 cannon in the nacelle and two more close to it in the wing roots, the S.M.92 had two cannon in the centre wing section, one cannon installed to fire through the hub of the right-hand engine, and two synchronized machine guns on each engine; while a fifth gun, firing aft, was installed on the tailplane. The S.M.92 was briefly flown by the Germans after the Italian surrender.

Probably the most promising of the Italian designs was the IMAM Ro 58, a clean all-metal design with twin tail fins, 1160 hp Daimler-Benz DB 601A-1 engines, and an armament of five 20-mm MG 151/20 cannon and a flexible 12.7-mm. The prototype made its first flight in May 1942 and was highly praised, but again the surrender of Italy in August 1943 prevented any production from taking place. It fate was thus the same as that of the last generation of Italian single-engined fighters, which were also compared favourably with German aircraft powered by the same engines, but were built in too small numbers to have any impact.

The Japanese Army

The concept of the twin-engined fighter held considerable appeal the staff of the Air Headquarters of the Japanese Imperial Army. For an organisation that was usually conservative in its approach to fighter design, it represented a radical step. However, Japan sought to pursue its interests over a vast geographical area, and the potential of a long-range fighter was evident to Japanese strategists. Such an aircraft would require the latest in aviation technology to be a success, a factor that stimulated the interest of a service that was still catching up with western developments, but also suggested a cautious approach. Hence, in March 1937 the Koku Hombu contacted the three major manufacturers, Nakajima, Kawasaki and Mitsubishi, and assigned them the experimental airframe designations Ki-37, Ki-38 and Ki-39. The specification that they were given was only for an interim twin-engined fighter design, intended primarily as an experimental aircraft, and it left much to be decided by the designer. It was challenging nevertheless, and Nakajima and Mitsubishi declined the submit a design.

The Kawaski Twin-Engined Fighters

Kawasaki undertook the project with more enthusiasm, and a mock-up of the Ki-38 had reached an advanced stage when in October 1937, the Army decided to stop work on the project and replace it with one around a more accurately defined specification. The primary task of the aircraft, it was now agreed, would be that of a long-range escort fighter. This resulted in a fairly light and maneuverable aircraft, with the relatively modest fixed armament of two 7.7-mm Type 89 machine guns and one 20-mm Ho-3 cannon, but fuel for 4 hours and 40 minutes at economic cruising speed. Top speed was to be 540 km/h at 3,500 m, in itself not particularly challenging in comparison to the speed of other twin-engined fighter projects initiated in this period, but difficult enough to achieve in combination with the range requirement. In December Kawasaki was instructed to update the project, which was now given the designation Ki-45, around the new specification. Kawasaki's new chief designer, Takeo Doi, put considerable pace behind the project and a prototype was ready in January 1939.

The Ki-45 was a mid-wing monoplane with pleasant lines and a crew of two, pilot and rear gunner. It had an elliptical wing platform, and rounded horizontal and vertical fins. Two 820 hp Ha-20-Otsu radial engines were installed in fairly short and stubby nacelles, which also housed the semi-retractable landing gear. The engines and nacelles turned out to be the weak point of the design; the Ha-20-Otsu was highly unreliable and the nacelles were a source of aerodynamic problems. Both performance and manoeuverability of the Ki-45 fell well short of requirements. Not without hesitation did the Army order an improved version with 1050 hp Ha-25 engines, which flew in the late summer of 1940. This at least raised performance, and hopes.

Meanwhile, Takeo Doi and his team had started on an extensive reworking of the design. Although the aircraft they were now preparing would enter service as the Ki-45-KAI, the abbreviation KAI meaning Kaizo, modified, it was in reality an almost entirely new design, with only the most superficial similarity to the original Ki-45. The cross-section of the fuselage was reduced. The wing abandoned the elliptical planform in favour of straight taper with rounded tips, and the engine nacelles were moved lower in relation to the wing and aerodunamically improved. The changes made the aircraft slightly larger, but more significantly, improved performance and made it easier to build. The Ha-25 engines were retained for the very first Ki-45-KAI aircraft, but were replaced by the more reliable Ha-102 later.

Where the Ki-45 had possesed pleasing lines conveying the general impression of a unfinished 1930s design, the Ki-45-KAI looked neat, crisp, and modern. Performance and handling were judged entirely satisfactory, and in early 1942 the Ki-45-KAI-Ko started to come off the production line, under the official designation of Type 2 Two-Seat Fighter. Armament was now two 12.7-mm Ho-103 machine guns in the nose, a 20-mm Ho-3 cannon in a ventral tunnel, and a flexible 7.92-mm Type 98 in the rear cockpit. This fell short of the firepower carried by competing twin-engined fighters, especially considering the low rate of fire of the Ho-3, but on the positive side the Ki-45 was the most manoeuverable twin-engined fighter of the war.

The operational roles of the Ki-45-KAI Toryu, which was given the name “Nick” by Allied Intelligence, soon diverged from the intended bomber escort role. The type was too much at a disadvantage in combat with Allied single-engined fighters. It was, however, an effective ground attack and anti-shipping aircraft and a good interceptor. When the need for a nightfighter arose, the Toryu was the logical choice, equipped with a pair of obliquely upward firing Ho-103 machine guns of 20-mm Ho-5 cannon instead of the upper fuselage fuel tank. To save weight, the Ho-3 cannon in the ventral tunnel was often removed. By day and night, the upward-firing guns made it possible to attack from below, where a bomber's defences were weakest.

This combat experience uses resulted in the development of adapted versions. Great confusion exists over the armament and specifications of the Ki-45 developments, but it appears that the Ki-45-KAI-Otsu was a specialized attack version, with the Ho-3 cannon replaced by a manually loaded 37-mm cannon, the Type 94. The Ki-45-KAI-Hei retained the Ho-3 cannon but omitted the fixed machine guns, to carry instead a 37-mm Ho-203 cannon in the nose. The Ho-203 fired a lighter projectile at a slightly lower muzzle velocity than the Type 94, and although automatic it fired at only 120 rpm. Such armament was unsuitable for combat against enemy fighters, but effective against ground targets or bombers. The Ki-45-KAI-Tei on the other hand adopted the 20-mm Ho-5 upward-firing guns of earlier improvised nightfighters as standard, in combination with the Ho-203 in the nose, but it still lacked radar. (A version with radar in the nose appeared near the end of the war, but probably did not see combat.) Another armament decision made in the final production runs was no longer to install the Type 98 in the aft cockpit, because it was ineffective.

While the Ki-45-KAI had been an indisputable succes, it had only been really sufficient for a short period. It had entered service in the middle of 1942, but from the second half of 1944 the Japanese needed an interceptor capable enough to provided a defence against the large, high-flying B-29. For this, the Toryu lacked the performance, although the heavy cannon of the later versions gave them more firepower than other Japanese interceptors. As it was one of the best aircraft Japan possesed for this task, an increasing number of Ki-45-KAI units was employed for home defence; but a better aircraft was urgently needed.

Kawasaki had not neglected to develop such an aircraft, but it had been delayed by doctrinal doubts on the success and correct employment of a twin-engined fighter. At first, Kawasaki started (on its own initiative and without army support) on a Ki-45-II development with a better streamlined fuselage and 1,500 hp Ha-112-II radial engines. At the suggestion of the Koku Hombu, the prototypes were completed as single-seat fighters, the first with the rear cockpit simply faired over, the second and third more thoroughly redesigned and fitted with an all-round vision canopy. The Ki-96 was an excellent aircraft, with a maximum speed of 630 km/h at 9,500 m and better handling than the Ki-45-KAI. However, when it flew in September 1943, the Army had already reversed its decision on the crew arrangement; therefore the goal of the Ki-96 programme was reduced to providing data for the two-seat Ki-102.

The development of the Ki-102 was given a high priority. Based on the Ki-96, this was an aerodynamically highly refined aircraft, planned from the start in two basic versions. The Ki-102-Ko would be a high-altitude interceptor with turbocharged engines and a 37-mm Ho-203 cannon and two 20-mm Ho-5; this was the aircraft that was needed to combat the B-29. And the Ki-102-Otsu was to be a ground attack model, lacking the turbochargers, and with a 57-mm Ho-401 instead of the 37-mm weapon. In parallel, Kawasaki was to work on the Ki-108, which reverted to the single-seat configuration, and would be equipped with a pressure cabin for high-altitude operations.

The first Ki-102-Ko was flown in June 1944, and following successful tests it was decided to phase out Ki-45-KAI production in favour of the Ki-102. However, while the aircraft was ready, the turbochargers were not, and therefore production started in October 1944 with the Ki-102-Otsu, officially designated Army Type 4 Assault Aircraft. Although a handful of interceptor models were completed before the end of the war, none of them was ever handed over to a combat unit. The bombing of the factories reduced even the output of the Ki-102-Otsu line to only 215. The prototypes of a Ki-102-Hei nightfighter version, with radar based on the German FuG 240 and two 30-mm Ho-155 cannon, never flew; both prototypes being seriously damaged by bombing before their first flight. The Ki-108 on the other hand did fly, but flight testing was also affected by bombing, and had not been completed at the end of the war.

Rob

One of the most daring attempts to build a twin-engined fighter was initiated by Kawasaki's Takeo Doi in 1939. Inspired by the engine installations of high-speed racing aircraft and developments in France, he projected a fighter that would be powered by two engines, one in the nose and one in the aft fuselage, each driving its own propeller on the nose — the aft engine driving the front propeller, but means of an extension shaft passing through the hollow gearing of the front engine. By combining two liquid-cooled Ha-40 engines (the Japanese licensed version of the Daimler-Benz DB 601) in combination with surface cooling, a speed of 700 km/h was expected to be within reach.

The technical problems and complications were obvious, and the Army did not give permission to start development work until late in 1940. The combination powerplant, designated Ha-201, was tested on the bench from the end of 1942 until the middle of 1943, and the prototype fighter was finally completed in November 1943. It was given the designation Ki-64, and the allied reporting name “Rob”. On the fifth flight a fire broke out in the rear engine bay, where cooling problems had been persistent, and although the pilot made a successful emergency landing, the sole Ki-64 never flew again.

Dinah

Mitsubishi had declined to submit the Ki-39 design as a heavy fighter, but it had served as the basis for a reconnaissance aircraft, the Ki-40. However, because this did not meet the army specifications for a long-range reconnaissance aircraft, it was abandoned and replaced by the Ki-46, which entered production just before the war in the Pacific broke out. As a reconaissance aircraft, the Ki-46 was a brilliant design, an elegant, streamlined aircraft with a high performance. The Ki-46-III model of late 1942 had a maximum speed of 630 km/h at 5,800 m, powered by 1250 hp Ha-112-II radials. Allied intelligence gave this excellent aircraft, which was able to outrun fighters, the reporting name “Dinah.”

This was well above the performance of the Ki-45-KAI and stimulated interest in a fighter version. The Ki-46-III-KAI, also known as the Army Type 100 Interceptor, featured a stepped cockpit window instead of the streamlined unstepped nose of the reconnaissance version. Armament consisted of two 20-mm Ho-5 cannon and a 37-mm Ho-204, mounted between the pilot's and observer's cabins, and angled 70 degrees up. A few of the older Ki-46-II were converted to incerceptors in the field, with the 37-mm Type 94 cannon. However, the Ki-46 had not been designed as a fighter; it was a lightly-built aircraft optimized to cruise at high speed and altitude. As fighter, it lacked the rate of climb, manoeuverability or sturdiness for its task. A version for ground attack, the Ki-46-III Otsu, would have been even more vulnerable, but only a few were completed.

An even unwiser improvisation delivered the Ki-109, a derivative of the Ki-67 Hiryu bomber. The Ki-67 was an excellent bomber, fast and very agile for its size. The Ki-109 was in the weight and performance class of a Bristol Beaufighter, although it was a considerably bigger aircraft. The intended use of turbo-supercharged engines promised to make the Ki-109 capable of intercepting the B-29. But the unsatisfactory state of development of the turbines forced the type into service without them, as the Ki-109-Ko, in November 1944. Without the turbochargers the aircraft was unable to get near enough to a B-29 to use its 75-mm Type 88 anti-aircraft cannon. Besides, this manually-loaded cannon and a 12.7-mm Ho-103 tail gun were its only armament, so that the Ki-109-Ko would have been woefully vulnerable to US escort fighters if it had encountered them. The Ki-109-Ko can only be considered a complete failure.

A much more promising Mitsubishi effort was the Ki-83, designed to meet a May 1942 requirement for a two-seat long-range fighter. The Ki-83 had simple, clean lines, with a streamlined fuselage and powerful Ha-211 turbo-supercharged engines. Armament would consist of two 20-mm and two 30-mm cannon in the nose. The aircraft was highly agile and the fastest Japanes fighter built during the war. Official performance figures included a top speed of 704 km/h at 9,000 m, but during post-war tests in the USA, using (much better quality) American fuel, it was good for 756 km/h. It hardly needs to be said that the development of the turbocharged engines was slow and plagued by problems, and the the fuselage and tail of the Ki-83 needed modifications too. The first prototype flew on 18 November 1944, but no production aircraft were delivered; in fact by the end of the war the emphasis had shifted to a specialized interceptor derivative, the Ki-103.

Another design that did not reach the frontline was the Ki-93. This aircraft was designed, not by a private manufacturer, but by the Army Aerotechnical Research Institute (Rikugun Koku Gijutso Kenkyujo, also known as Giken). Work on the drawings started already before the Pacific War, but in part due to changes in the tasks assigned to the aircraft, and in part to the inexperience of the Giken in aircraft design, a prototype was not flown until 8 April 1945. Like most Japanese fighters of its generation, the Ki-93 was elegantly streamlined; the six-bladed propellers of the two 2,700 hp Ha-214 radials emphasised the promise of high performance. In fact its predicted performance was good but not impressive, and the real strong points of the Ki-93 were ruggedness, extensive armour protection, and heavy armament: a 57-mm Ho-402 cannon in the nose, a 20-mm Ho-5 in each wing root, and a flexible Ho-103 in the rear cockpit. In other times could have been a successful fighter-bomber and close-support aircraft, but in reality its first flight was also its last one, for the prototype was damaged during the landing and subsequently destroyed by bombing.

In summary, the experience of the Japanese Army with its twin-engined fighters was not a very happy one. It managed to enter the war with the Ki-45-KAI, which was a sound, versatile design, but like many Japanese designs it aged quickly and in the second half of the war it was no longer a match for the enemy. Attempts to replace it by a more modern aircraft all foundered, both for technical and for institutional reasons. The Army changed its mind on the features such an aircraft should have a few times too often, and the Japanese industry never brought to an acceptable standard of reliability the powerful, high-altitude rated engines that were so much needed.

The Japanese Navy

Lack of cooperation between the Imperial Japanese Army and Navy was one of the most serious weaknesses of the Japanese war effort. In agreement to the same tortuous logic that caused the Army to operate its own submarines, the Navy developed several categories of land-based aircraft, distinct from army types. For the history of the twin-engined fighter this is perhaps fortunate; there was little room for twin-engined types on an aircraft carrier.

In June 1983, the Navy formulated a requirement for a twin-engined long-range escort fighter. At that time, of course, work on the Ki-45 was already in progress, and the Navy had kept itself informed on this project. But the specification for the “13-Shi twin-engined land-based fighter” was somewhat different, in that the Navy wanted a three-seat rather than a two-seat design, and the range requirement of 2400 km (3700 km with auxiliary fuel tanks) was well above what the Ki-45 was capable of. On the other hand, the top speed of 520 km/h was below that specificated for the Ki-45, and armament specifications were almost identical. Perhaps the Navy really wanted to avoid purchasing a version of the Ki-45, for it submitted its request only to Nakajima and Mitsubishi. And again, Mitsubishi declined to participate.

The Nakajima J1N became a bigger and somewhat more complex aircraft than the Ki-45, but despite Nakajima's best efforts, it was not a very successful one. To meet the range requirement and have a sufficiently low wing loading for high agility, the type was given a wing span of 17 m, equipped with automatic “combat flaps”. But on tests, the manoeuverability of the J1N was found to be insufficient for a fighter. The engines were to be Nakajima Sakae 14-cylinder radials, rotating in opposite directions, the upper blades moving inwards. But the modified gearing in the right-hand Sakae 22 engine reduced its power, and finally the production aircraft were equipped with two Sakae 21 engines, both turning in the same direction. The sophisticated rear defence system of the J1N was one of its most innovative elements. The cockpit fairing ended in two small remote-controlled turrets, each with a pair of 7.7-mm Type 92 Model 3 machine guns. The turrets were staggered, the second one behind and below the first one, and a sliding cover reduced their drag when not in use. Unfortunately, the complexity of this system delayed the delivery of the aircraft, and ultimately it was never perfected; the aiming system remained too slow and inaccurate. Only a small number of J1Ns were equipped with it, but meanwhile, its weight helped to reduce the performance of the J1N, which was found to be below specifications.

Tests following the first flight on 26 March 1941 revealed these and other defects, and as a result the future of the J1N became uncertain. The aircraft was considered unable to operate as a day fighter, and Navy's carrier-based fighter, the famous A6M “Zero”, possessed an impressive range that removed the need for a dedicated escort fighter. The story of the J1N would probably have ended with its condemnation in the summer of 1942, if the Navy had not realized at that time that its reconnaissance aircraft, the Mitsubishi C5M, was obsolete, and that the J1N might replace it. The result was the “Type 2 land-based reconnaissance aircraft”, or J1N1-C.

The J1N1-C lacked the fixed 20-mm cannon of the fighter prototype, but intially the pair of 7.7-mm Type 97 machine guns in the nose was retained, although it was removed from later models. The rear armament was also progressively modified, first by replacing the troublesome remote-controlled turrets by a simple flexible gun, and at the end of 1942 by installing an enormous and highly drag-inducing manually-operated turret with a 20-mm cannon, taken from the cancelled G5N heavy bomber. In the middle of 1943 a somewhat more streamlined turret was introduced, but the J1N1-C was hardly a success. It was clearly inferior to the Army's Ki-46 reconnaissance aircraft, which began to be used also by the Navy.

The J1N was rescued from oblivion by the great enthusiasm of a Navy officer, Commander Kozono, for obliquely-firing guns. Cdr. Kozono Yasuna appears to have been rather eccentric and some of the gun installations proposed by him were useless, but his installation of oblique-firing cannon in the J1N made a qualified success out of an indifferent aircraft. The first two modifications consisted of two 20-mm Type 99-2 cannon aimed 30 degrees up, and two more 30 degrees down, all installed in the space once taken by the remote-controlled gun turrets. Matching sights were installed in the cockpit. Trials showed the value of the installation, especially for the interception of bombers at night, and the modified aircraft went into production as the J1N1-S Gekko (Moonlight). From the 301st aircraft onward, the complicated fuselage decking for the gun turrets was finally omitted, and the cannon installed in a simpler and better streamlined fuselage structure.

Armament installations varied locally, as older aircraft were modified to J1N1-S configuration, or J1N-1S were adapted by installing or removing cannon. A few came out of the factory as J1N1-Sa models with three upward-firing guns instead of two. The downward-firing cannon were often removed, as they were not very useful and the Gekko needed to be light to stand a chance of intercepting a B-29. A major step foward could have been the first installations of radar on a Japanese fighter, inspired by captured American radars, but by 1945 the Japanese industry was seriously troubled by lack of resources and the manufacturing quality of these radars was apparently poor enough to render them useless.

Overall, the J1N1-S proved to be a fairly effective nightfighter, and its victims included two dozen B-29s. Whether this really justified the existence of a separate fighter type in parallel with the Ki-45, can be seriously doubted.

The Navy imitated the Army in more than one way, for it too failed in its effort to find a modern replacement for its twin-engined fighter. The Nakajima J5N Tenrai, first flown in July 1944, had poor handling qualities and, despite the use of 1990 hp Homare 21 engines, disappointing performance; it achieved a top speed of only 597 kmh at 6,000 m. Four prototypes were completed as single-seat fighters and two as two-seaters before the inevitable cancellation. Aichi contribution a dedicated nightfighter design, the heavily armed S1A Denko, but at the end of war its prototypes had not yet been completed.

The Navy did have a bomber that appeared to be highly suited for a nightfighter conversion. The P1Y had been designed by the Yokosuka naval arsenal as a fast medium bomber for use against naval targets, capable of level attacks from altitude, dive bombing, and torpedo bombing. The P1Y was compact for a bomber, with a wing span of 20 m. It carried only only a moderate bomb load or a single torpedo, and was fast enough to outrun fighters at low level. When first reported by Allied intelligence, the type was assumed to be a fighter and accordingly given a boy's first name as reporting name: Francis. When the aircraft was discovered to be a bomber, this could be changed to Frances without creating confusion. The potential of the type was, however, balanced by its complexity and difficult maintenance.

A number of P1Y1 bombers, powered by 1825 hp Nakajima NK9C Homare 11 radials, were modified to P1Y1-S nightfighters, equipped with a pair obliquely upward firing 20-mm cannon in the front fuselage and a second pair in the aft fuselage. They were given the popular name Byakko (White Light). The P1Y2-S version, powered by more reliable 1850 hp Mitsubishi Kasei 25 engines, was known as Kyokko (Aurora), and was factory-produced by Kawanishi, rather than a bomber conversion. The P1Y2-S possessed an AI radar, of which the antenna array was installed on the nose. Besides two upward-firing 20-mm cannon in the aft fuselage, the P1Y2-S had a single flexible 20-mm or 13-mm weapon to defend the rear, and it retained the internal bomb bay for intruder missions.

But neither version turned out to be very useful for the Japanese. The Kyokko made its first flight in June 1944, and when it entered service the biggest threat was the B-29 bomber. Like most Japanese fighters of the period, the P1Y2-S lacked the performance to be an effective B-29 interceptor. The original bomber model had been fast, but it had been fast at low altitude, where its missions and its targets were to be found. The nightfighter conversion failed because the performance at high altitude was disappointing. In addition, we can consider that the armament of the P1Y-2S was too light and its radar probably insufficiently reliable. It is reported that most of the small number (96) of P1Y2-S nightfighters built by Kawanishi were converted back to bombers, with removal of their radar and fixed guns. The limited number of P1Y1-S conversions also failed to make an impression.

Thus the IJN's efforts to develop twin-engined fighters produced one qualified success, the J1N1-S, and a series of failures. At the end of the war, the Japanese army could at least show a series of impressive prototypes, although to little benefit for themselves. The Navy did not even have that much to show for its efforts.

Perhaps the Japanese experiences demonstrate how much the success of the twin-engined fighter concept depended on technical advances, and especially on powerful engines. Japanese aircraft designers had to work with engines of generally lower power and inferior reliability, compared to those fielded by Japan's enemies. As the war progressed and took a turn for the worse, the ability of the national industry to manufacture armaments of advanced technology and high quality entered a terminal decline. The result was that impressive prototypes were doomed to remain impressive prototypes, for Japan was unable put them into service.

Soviet Luxuries

The Japanese experience would have been understandable to Soviet policy makers and designers. For most of World War II, the USSR regarded twin-engined fighters as a luxury it could not afford. Given the focus of the authorities on tactical air warfare, this might be considered a bit surprising. One could very well imagine a useful role for a heavily-armed twin-engined fighter-bomber over and behind the battefield, especially over a battle area that was as vast as the Eastern Front. And the USSR's only twin-engined fighter in service, the Pe-3, actually found a lot of use in this type of mission, but only a few hundred of these aircraft were completed. A combination of small single-engined fighters and single-engined attack aircraft was preferred, with a modest investment in twin-engined light and medium bombers. And this despite a long-running effort to design heavy fighters, that has achieved some excellent results.

Tupolev

Despite the isolation of their country in the late 1920s, Soviet policy makers appeared to have been as much influenced by the international fashion for twin-engined fighters as anyone else. In these years, the USSR was building a strategic, long-range air force, and it possessed a fleet of ANT-6 (military designation TB-1) heavy bombers that, by the standards of the time, was very impressive indeed. One of the roles envisaged for a twin-engined fighter was long-range bomber escort, but it was also expected to fly independent “cruiser fighter” missions deep into enemy territory, and to perform reconnaissance, bombing, and torpedo-bombing.

Andrei N. Tupolev suggested that the requirement could be met by a scaled-down version of the ANT-6. This made more sense than one would think, for in an age of biplane fighters with mixed structures, the huge monoplane ANT-6 had been a technological leap forward. His proposal was accepted, and in September 1929 the ANT-7, designated R-6 by the military, made its first flight. It had the familiar Tupolev trademarks: Thick monoplane wing and rectangular fuselage, corrugated metal skinning, fixed landing gear, and open cockpits. It was powered by liquid-cooled M-17 engines, a license-built version of the BMW VI. Production began in 1930 and ran for four years in a number of different versions, but at the end of that period the aircraft was already obsolete, overtaken by the rapid technological development of the 1930s. The KR-6 “cruiser recconaissance” version had a top speed of 226 km/h at sea level, needed no less than 13 minutes to climb to 2000 m, and had a range of 1480 km. The armament consisted of open nose and dorsal gunnery positions with two 7.62-mm DA guns each. Other versions (but not the KR-6) also had a retractable ‘dustbin’ below the fuselage with a single DA. Basically, this was just the standard bomber armament of the period.

In 1932 the prototype of a replacement for the ANT-7 had been ordered, the ANT-21. The ANT-21 initially re-used some parts of the ANT-7, including the engines and some structural components in the wing, but it featured a new semi-monocoque fuselage, smooth instead of corrugated metal skinning, and retractable landing gear. These refinements were amply rewarded, for the top speed at sea level rose to 351 km/h. An accident during flight testing resulted in the construction of a much-modified ANT-21bis, also known as the MI-3D, with M-34N engines. However, the type was not accepted for production.

At this point in time, Soviet development of twin-engined fighter was influenced by a new armament option. In the early 1930s Leonid V. Kurchyevskii developed a family of recoilless guns, building on the earlier designs by the American Cleland Davis. The Davis and Kurchyevskii designs solved the recoil problem by firing two projectiles: A shell forward, and a counterweight to the rear. The recoil forces generated by the firing of the two projectiles compensated each other. The concept produced in a long gun, with muzzles at both ends and a complicated breech mechanism in the middle; all very inconvenient for a traditional fighter. However, the elimination of recoil made it possible to install 37-mm, 76-mm, and even 120-mm guns in a relatively small aircraft. A specially designed twin-engined fighter was thought to be the best candidate to carry such a weapon.

One of the possibilities was demonstrated by the ANT-23 fighter, a small twin-boom design with a tractor and a pusher engine in its short fuselage. The unique feature of the ANT-23 was that the tail booms were actually formed by the barrels of the 76.2-mm APK-4 cannon. Despite its unusual configuration the aircraft flew well, but it was overtaken by other developments.

The ANT-29, also known as DIP, appears to have been broadly based on the ANT-21bis, but had a short, deep and narrow fuselage designed to house two large APK-8 recoilless guns in its bottom, with the muzzle of the barrels protruding from the nose and a counterweight fired from the tail. In relation to the compactness of its fuselage the ANT-29 had a huge 56.8 square meter wing, and the engines were imported French Hispano-Suiza 12Ybrs V-12s of 760 hp. The ANT-29 flew late in 1935, but was not accepted for service. Nevertheless, the work on the ANT-29 was not entirely wasted: The aircraft was closely related to the ANT-40 or SB, which was the most important bomber of the late 1930s and represented a quantum leap forward in the design of combat aircraft when it entered service.

And fighter development in this family had not yet been abandoned. The development of the ANT-46 or DI-8 was allowed to continue in parallel with that of the ANT-49, and it was flown in 1936. The DI-8 was armed with a 37-mm APK-11 cannon buried in each wing outside the propeller arcs, plus four ShKAS machine guns in the nose. However, 1936 was also the year in which the great Stalinist purges began. And the victims of the purges were found not only among political and military leaders, but also included factory managers, scientists and engineers. Leonid Kurchyevskii disappeared. Andrei Tupolev and many of the members of his team were arrested. As a sign of the times, the accusations against him included the absurd statement that Tupolev had supplied the Germans with the plans for the Bf 110.

Petlyakov

One result of Stalin's reign of terror was that many Soviet aircraft designers found themselves imprisoned in design and study offices controlled by the NKVD. Many Soviet combat aircraft were designed by what was in effect slave labour, engineering offices filled with a captive staff that in some cases had a suspended death sentence looming over them. Vladimir Petlyakov, formerly one of Tupolev's closest associates, thus became the engineering lead of an imprisoned group of fifty people tasked with the development a twin-engined fighter. (The formal head of the design bureau was an NKVD officer.) They were ordered to design a highly ambitious high-altitude, long-range fighter, intended both as an interceptor and as an escort fighter for the new ANT-42 bomber. The specifications called for a top speed of 630 km/h at 10,000 m, with an operational ceiling of 12,500 m.

The VI-100, which made its first flight on 22 December 1939, certainly looked ready to meet this demanding specification. It had an elegantly streamlined fuselage, almost a monococque structure with its strength derived from a relatively thick skin attached to the fuselage frames, without supporting stringers. The wing carried the two M-105 engines in front of the leading edge, in neatly streamlined nacelles that also contained the TK-2 turbosuperchargers and the wells for the main landing gear. The superchargers also provided the compressed air for the crew of three in two pressure cabins, fore and aft. The fixed forward-firing armament consisted of two 20-mm ShVAK cannon (300 rpg) and two 7.62 mm ShKAS (900 rpg). The planned use of a pressure cabin made it hard to provide flexible guns for the defense of the rear, so the installation of a single fixed rearward-firing ShKAS was contemplated, although not proceeded with. The first prototype could also carry bombs under the wings, while the second prototype even possessed an internal bomb-bay.

The VI-100 had important teething troubles, and was slightly slower than has been hoped, but nevertheless the design was regarded a success. However, its intended purpose was to be changed abruptly. In 1940, it became apparent that the potential enemies of the USSR would not be putting into service high-altitude bombers at any time soon. In that context, the production of the complex and expensive VI-100 was not justified. However, the VVS urgently needed a new tactical low-altitude bomber to replace the obsolescent SB. Hence it was decided to use the VI-100 as the basis for the development of a twin-engined dive-bomber, the PB-100.

The VI-100 was not cancelled outright, but the relatively low priority accorded to it inevitably resulted in its neglect. For the changes required to create the bomber version were very substantial, and only by great exertion was it possible to fly the first PB-100 on 15 December 1940. It entered service as the Petlyakov Pe-2, which soon proved itself an excellent light bomber. A total of 10,574 were delivered to the VVS during the war.

This left the USSR without a twin-engined fighter, and soon after the opening of the war with Germany a need was felt for just such an aircraft, mostly as a nighfighter. As the Pe-2 was the most suitable of all available aircraft, the conversion of one to a fighter prototype was ordered — and reportedly only seven days passed between the formulation proposal and the completion of its state flight tests. The new fighter was known as the Pe-3.

Of course, development in such a short time was only possible because very limited modifications were allowed. The fuel tankage was increased, the crew reduced to two, and the armament slightly increased: The production Pe-3 initially had two fixed UBK 12.7-mm machine guns with 250 rounds each, a ShKAS in the turret at the rear of the cockpit, and (because the ventral gunner had been omitted) a fixed ShKAS on a ventral mount to deter attackers. This was clearly too light, and fixed armament was soon increased by installing a 20-mm ShVAK cannon, while the flexible ShKAS was replaced by a 12.7-mm UBT. With some other changes, this resulted in the updated Pe-3bis. A later change relocated the machine guns from the nose to the former bomb bay. As could be expected for a hastily improvised fighter, the armament installation of the Pe-3 was never entirely satisfactory. In the summer of 1941 a more through fighter modification, the Pe-2I, was ready for testing, but despite better performance and a better armament installation, it was not accepted.

Because the Pe-2 had possessed excellent performance, the Pe-3 could match a Bf 110C in speed, though not in manoeuverability, and it was lightly armed for a heavy fighter. It also lacked basic equipment for the nightfighter mission; the initial production runs had poor radio and not even a radio compass. Only 360 Pe-3s were completed, and they were more useful as long-range reconnaissance aircraft and ground attack aircraft than as fighters.

Yakovlev I-29

There was another light bomber that held, at least on paper, some potential as a twin-engined fighter. Yakovlev's BB-22, later called the Yak-2, was a fast light bomber. The prototype made its first flight in early 1939, and the BB-22 short-range, fast bomber version entered production in 1940. A highly streamlined design, it was greeted as something of a miracle by the Soviet leadership, but it turned out to be a very disappointing combat aircraft. The high speed of the prototype, 567 km/h at 9,900 m, had been achieved by sacrificing operational equipment and armament. Adding these reduced speed by over 50 km/h, and the handling characteristics left something to be desired, too. Worse, the production aircraft was barely able to carry 400 kg of bombs.

From the start, the aircraft had also been intended as a fighter, but characteristic for the initial concept was that the proposed fixed armament was just a single 20-mm ShVAK, replacing the fixed 7.62-mm ShKAS of the bomber version. This was later revised to a more realistic pair of ShVAK, a single ShKAS in the lower nose, and two ShKAS more installed between the cylinder banks of the engines, firing through the propeller hubs. The compensate for the weight increase, the crew of the fighter version was reduced to one.

The highest priority was given to the bomber version, and then to the R-12 reconnaissance model; so an actual fighter prototype was not flown until December 1940. This I-29 was powered by M-105 engines and was armed with two ShVAK cannon in the lower fuselage. However, with the evacuation of the aviation industry to the East, beyond the reach of the German forces, work on the I-29 was halted.

Polikarpov

In 1935, the Nikolai Polikarpov started work on the development of a twin-engined multi-role aircraft. After what appears to have been considerable debate on the intended role of such an aircraft, with a related series of designation changes, a prototype finally emerged in 1937 as the VIT-1, a designation indicating that it was a Vozdushny Istrebitel Tankov, or anti-tank fighter. It was a three-seater powered by two 960 hp M-103 engines, and its most striking features were the two long barrels of the powerful ShK-37 cannon installed in the wing roots. A 20-mm ShVAK in the nose, with a limited range of movement, and a 7.62-mm ShKAS in the rear cockpit completed the gun armament. An internal bomb bay and external bomb racks were the other part of it, for the VIT-1 was also expected to act as a dive bomber.

The VIT-1 was a good basis for further development, but more powerful engines were required to improve the top speed (450 km/h) and its take-off and landing characteristics. Hence the development of the VIT-2, with 1050 hp M-105 engines and distinguished most easily by its twin tail fins, but in fact extensively redesigned and provided with heavier armament. The aircraft was ready in the spring of 1938 and with M-105 engines installed, the VIT-2 prototype reached a speed of 513 km/h at 4500 m. However, the type still suffered from unreliable engines and numerous aerodynamic problems, and needed considerable modifications to rectify the worst of them. It was early 1939 before the VIT-2 finally passed its state testing and was recommended for production. But it was not to be. Limited industrial resources, political disagreements, and a preference for the SPB dive-bomber version of the same airframe, all led to the abandonment of the project.

Of course, even if the projected VIT-2S production version had been built, it would have been a ground attack aircraft with perhaps a limited air combat capability. But even before the type fell by the wayside, Polikarpov was asked to design a proper twin-engined fighter, intended as a heavy escort fighter, the TIS. Work on the VIT and SPB delayed progress on the TIS, and the first flight of the TIS(A) prototype was made in September 1941. It was smaller than the VIT, but nevertheless considerably heavier, despite the use of all-metal instead of mixed construction. This was a characteristic of several Soviet twin-engined fighter designs, and the result of building them to very high stress factors, up to loads of 12 g. Of course, the additional weight hampered performance, and especially take-off and landing from rough fields.

But the TIS(A) had very clean, elegant lines, with closely cowled 1400 hp Mikulin AM-37 in-line engines and twin tailfins. The wing had a broad chord at the root and tapered sharply towards the tips, mostly on the trailing edge. Top speed achieved during tests was 555 km/h at 5800 m, but clearly the AM-37 engines were far from ready for service use. The German attack, the evacuation of the design offices to the East and the urgency of other work caused work on the TIS to be temporarily laid aside, until the second half of 1943. In June 1944 flight testing began of an improved TIS(MA), intended to have AM-39 engines, but in fact equipped with much more readily available AM-38F power plants. With these low-altitude rated engines it was capable of 535 km/h at 1650 m. As impressive the aircraft could have been in 1941, by late 1944 there was little future and no operational need for the type; and after the death of Polikarpov in July 1944 his design bureau was closed down.

The original TIS(A) had been armed with four fixed 7.62-mm ShKAS machineguns in the upper nose, two ShVAK cannon in the wing roots, and two more ShKAS guns on a dorsal TSS-1 and a ventral mount. The latter two were both controlled by the observer, for the TIS was a two-seat aircraft. The ventral gun was rather impractical, for its use required the observer not only to leave his seat, but to squeeze his way to under the armour plate that protected his station. It was abandoned on the TIS(MA); the dorsal mount was replaced by a more powerful VUB-1 with a 12.7-mm UBT but the biggest change was made to the nose armament. Two 12.7-mm UBS were installed in the nose instead of the four rifle-calibre machine guns, and two Sh-37 37-mm cannon were placed in the wing roots. A ventral pack could accomodate a NS-45.

Mikoyan-Gurevich

Of the several Soviet twin-engined fighters that got close to production, one of the most interesting ones was the DIS-200, designed by the new OKB or design bureau constituted by Artyom Mikoyan and Mikhail Gurevich. (Both, incidentally, managed to stay of Stalin's jails.) Their initial concept included the use of the new powerful diesel engines then being developed Charomskyi. Diesel engines may sound like an unlikely choice for a fighter, but the advantage of such power plants was that they were fuel-efficient, and on paper allowed the aircraft to have a long range. However, these diesel engines never matured sufficiently, and the first prototype had conventional liquid-cooled Mikulin AM-37 V-12 engines, while the second switched to M-82F radials.

The DIS-200 was a compact single-seat fighter, with a wing span and length close to that of the Fw 187. It was considerably heavier, however, and wing loading was high. The wing had a very deep chord at the wing root, and tapered sharply towards the tips. To reduce take-off and landing speeds the aircraft was given generous flaps on the trailing edge, as well as ailerons that could be lowered together with the flaps. The engine nacelles were slung below the wing, which in turn was attached to the bottom of the slender, short-nosed fuselage. The pointed engine cowlings were reminiscent of that of the MiG-3, with prominent oil coolers installed at each side. The radiators for the engine coolant were narrow slots, also on each side of the engine cowling, with an exhaust at the rear of the cowling near the wing trailing edge. The exhaust was ducted to over the wing leading edge.

The bubble cockpit and inverted gull shape of the wing gave the pilot a good view foward and in the upper hemisphere, but he could look downwards only through glass panels in the bottom of the nose. The wing roots carried a fixed armament of two 12.7-mm UBS and four 7.62-mm ShKAS guns. Options for armament fitted under the fuselage included a pack with a powerful 23-mm VYa-23 cannon for the fighter role, a torpedo for anti-shipping missions, or a bomb. (The second prototype, with more powerful engines, could reportedly carry a pack with two cannon.)

In weight and performance, the DIS-200 was close to the Mosquito. It was, incidentally, also built largely of wood, to save on scarce light alloys. High performance and multi-role versatility could have given it a formidable potential. However, the first prototype had the misfortune to fly at the end of May 1941; the German attack in the next month upset all planning, and the notion of producing the aircraft in series as the MiG-5 had to be abandoned. As in the case of the TIS, the type was also handicapped by its underdeveloped and unreliable AM-37 engines.

The DIS was not abandoned. In late 1941 the design team was reinforced with Petr Grushin, whose own Gr-1 twin-engined fighter had been destroyed by German ttack before its first flight. (Very little is known about the Gr-1, but sketches show an aircraft remarkably similar to the Bf 110.) The second DIS-200 prototype, completed after the OKB had been evacuated to the East, flew in January 1942, with better developed 1,700 hp M-82F radials, in close-cowled nacelles and driving four-bladed propellers. This was well ahead of the TIS(MA), but nevertheless too late; the VVS had already settled on a policy of tactical air support and no longer needed a long-range fighter.

The Unfortunate

The Polikarpov and Mikoyan-Gurevich bureaus found a competitor in the OKO team, lead by Vsevolod Tairov. His OKO-6 design was ambitious and radical. Of mixed construction, with a relatively generous use of duralumin and magnesium alloy, the OKO-6 was very compact, with a wing span of 12.65 m and only 25.4 square meter wing area. It had a deep but narrow and short fuselage, ending in a single tail fin; the rather plump engine nacelles housed M-88 radials, cooled through narrow slots behind large spinners. The propellers rotated in opposite directions. Armament consisted of four ShVAK cannon in the fuselage bottom and two ShKAS in the upper nose decking.

The OKO-6 looked like a recipe for aerodynamic problems, and indeed tests, following the first flight on the last day of 1939, revealed a need for substantial changes. A year passed before a modified OKO-6bis flew, also known as Ta-1 in honor of its designer, with a longer fuselage ending in twin tail fins, besides a series of other improvements. But an engine failure resulted in a crash, and again several months were lost before the first prototype had been rebuilt. This was brought to OKO-6bis configuration but with M-89 engines, and renamed Ta-3. In this form, the aircraft was said to posses better handling than the MiG-3. This was very faint praise indeed, but the type's high performance and heavy armament still resulted in a recommendation for production. However, a requirement for a considerable range increase triggered yet more delays, and it was May 1942 before the Ta-3bis (with fuel tanks in the magnesium alloy outer wing panels) entered official tests.

By August 1942, two and a half years had passed since the first flight of the OKO-6, but aerodynamic problems persisted and the M-89 was still too unreliable for production. Designer Tairov himself had been killed in an air accident in December 1941, and the interest in twin-engined fighters had waned. The aircraft was abandoned and forgotten.

Loss

And so, overall the Soviet effort to develop twin-engined fighters turned out to be a waste of time. Despite a surprisingly large number of prototypes: The VI-100, I-29, VIT-2S, TIS, DIS-200, Gr-1 and Ta-3, the only twin-engined fighter to see service was the improvised and rather unsatisfactory Pe-3. This was operated on a limited scale and mostly in a light bomber role. After the war some nightfighter developments of the Tu-2 bomber appeared.

Some technical reasons can be pointed out. In the story of the aircraft above, engine troubles are a constant. A good twin-engined fighter needed the most powerful available engines, but Soviet engine development during the war years largely failed to make new designs sufficiently reliable. The M-105 was too small for a heavy fighter, and the M-82 reached a sufficient state of development in late 1942, when it was already too late. The AM-38 was a good engine, but heavy and rated only for low altitudes. Without a good turn of speed, a twin-engined fighter flying at low level was very vulnerable to being bounced.

Other factors helped to eliminate the operational need. As nightfighter radars were not developed until after the war, there was evidently no need for a nightfighter able to carry one. Operations over sea were mostly limited to areas where the enemy had no fighter bases, so there was no requirement for a long-range fighter.

In the end, however, it was a doctrinal choice. The VVS chose not to deploy such aircraft, and as long as it did not want to operate deep over enemy territory, in did not need to. There was always the possibility of constructing airfields close to the front. One can speculate that tactical air operations over the vast area of the Eastern Front would be an excellent environment for a fast, long-range fighter and fighter-bomber, striking at gathering areas and transport infrastructure behind the enemy front lines. However, for most of the war the USSR could rely on partizan forces to make the hinterland unsafe for the enemy.

The American Effort

Whatever limitations the Soviet twin-engined fighters had, from a tactical point of view all of them made some sense, with perhaps the exception of the Yakovlev I-29. The same cannot be said of the US efforts in the same direction. Perhaps the operational experience of Russian participants in the Spanish Civil War played a role in keeping the wilder ambitions of Soviet designers in check. However, one gets the feeling that many more outlandish concepts were studied in the USA for the simple reason that with all the technology available there, much more was possible. Of course these white elephants would not have been created the air force had not allowed, and even encouraged it.

Bell FM-1 Airacuda

In the early 1930s, the Army Air Corps was highly conservative. In 1934 it adopted its first monoplane fighter, the Boeing P-26, but this was still very traditional in more ways than one. Not only had it wire-braced wings and fixed landing gear, but it had the dimensions and weights of a traditional biplane fighter; it was small, light, and powered by a light engine. The P-26 was too slow to catch modern bombers and too weakly armed to shoot them down.

Nevertheless, in 1934 the Materiel Division initiated a study of a heavy "multiplace fighter" design, which centered around the concept of a Martin B-10 bomber modified to operate as escort fighter, or as the documents quaintly put it, "a bombardment accompanying weapon". This study mainly demonstrated that this was a poor concept. The Martin B-10 would only be slowed down by adding extra armament and reinforcing it for combat manoeuvering, making it slower than the bombers it would escort. Besides, the basic design was already ageing rapidly.

This could have been the end of it, but the designer Bob Woods had been considering a much more advanced multiplace fighter, and Larry Bell successfully promoted it to the air corps. They key elements of the Bell Aircraft Corporation offer were the new Browning-designed 37-mm cannon and Allison V-1710 V-12 engine. The Allison engine, in its turbosupercharged form, offered high performance at the expected cruising altitudes of long-range bombers. The cannon would allow enemy bombers to be destroyed from a safe distance, well out of reach of their defensive armament. For this purpose, they would be installed on powered mounts, and aimed by an automatic fire control system. A specification X-604 was written around this concept, and Lockheed was invited to prepare a competing design. But it was the Bell aircraft that won the order, by a narrow margin.

The aircraft was given the designation FM-1 for Fighter, Multiplace, at a time when single-seat fighters were still given P designations for Pursuit. Several layouts had been studied, with up to four cannon, but the Bell FM-1 Airacuda emerged with two V-1710 engines driving pusher propellers, in large nacelles that each also contained the powered gun mounts and gunner's stations in front. The main role of the gunners was to feed the cannon, for the weapon that was used did not have the belt feed that characterised the M4 cannon as later used in the P-39. A .30 Browning machine gun was installed co-axially with the cannon to aid in aiming, but visual gunlaying from the nacelle would still have been poor, also because the view from them was rather restriced.

Instead, the guns were aimed from a station in the main fuselage. The fire control system was based on systems developed for laying anti-aircraft guns, and was quite sophisticated for the time. Inputs from an optical sight and a rangefinder were processed by a computing unit, and then the aiming parameters were electrically signalled to the nacelles. There, hydraulic servo controls aimed the guns as desired.The movement was restricted to a 30 degree cone in front of the aircraft, so the Airacuda was to fly behind the enemy bomber formation, supposedly picking off its targets one by one.

All this came at a cost, and the XFM-1 prototype, flown in July 1937, possessed a 21.33 m wing span, an empty weight of 6,200 kg, and room for a crew of five. With V-1710-13 engines that delivered only 1,090 hp each, considerable less than had been hoped for, the top speed was only 434 km/h at 6,100 m, well below the original specification. Nevertheless, the prototype was followed by 13 service test aircraft under the designation YFM-1 — actually eight YFM-1 aircraft with turbosupercharged V-1710-23 engines, three YFM-1A with turbocharged engines and nosewheel landing gear, and two YFM-1B with tail-dragger landing gear and V-1710-41 engines without the turbo. These aircraft had, besides the cannon and their co-axial .30 machine guns, two hatches in the sides of the aft fuselage mounting .50 Browning machine guns, a retractable dorsal turret mounting a .30, and a belly fairing with a .30 at the end.

The YFM-1s were very little flown, totalling less than 500 flying hours between them before the survivors were disposed of. They were highly specialized aircraft, with a dubious operational role against a threat that turned out to be non-existent; the continental USA was well beyond the range of enemy bomber fleets. Their real value was that of an exercise in design and engineering of advanced combat aircraft, not in actually operating as such.

Skyrocket

Not only in the army, but also in the navy, brains were kept busy considering the defense of the USA against enemy bombers. The naval mind was of course much concerned with coastline targets, naval bases and ships, which could be attacked with little warning. To effectively defend such targets against attack by modern high-altitude bombers, a fighter needed heavy armament and a very high climb rate. In 1935, it appeared to the Bureau of Aeronautics (BuAer) that the power of the available engines would not allow single-engined fighters to meet the requirements of this role, even allowing for engines then under development. And carrier fighters, of course, would be handicapped by their need for extra structural strength and arrester gear.

The result of this line of thought was, in 1935, BuAer Proposal SD-24D for a fast-climbing naval interceptor, followed in 1938 by Specification SD112-14. The Navy's emphasis on climb and the requirement to operate the aircraft from carriers of course ruled out elephantine concepts such as the FM-1. What it wanted was a small, twin-engined, single-seat interceptor with excellent performance. The design that the Navy picked to be built and flown was the Grumman G-34, and the prototype was given the official designation XF5F-1.

This was about the smallest aircraft that could be built around two powerful radial engines. To be able to install the engines as close together as possible, the fuselage of the XF5F-1 was given a short nose, and after wind tunnel trials it was cut back further, so that it did not even extend as far as the leading edge of the wing. The engine installation not only reduced wing span, its reduced asymmetric effects when flying on one engine, and allowed for smaller tail fins. The downside of this configuration was that much of the aircraft was now behind the center of lift of the wing; the engines themselves contributed only about a third of the total empty mass. This made it difficult to get the center of gravity of the XF5F-1 right. It also gave the aircraft unique looks, with a noseless fuselage, radial engines in stubby nacelles, and angular twin tail fins.

The engines were a problem. The two options were the Wright Cyclone, which had its cylinders arranged in single row, and the Pratt & Whitney Twin Wasp, a two-row design. Their respective designations R-1830 and R-1820 indicate that the engines were close in internal volume, as they were in weight and power; but the Pratt & Whitney design had a smaller diameter and offered less drag. The Navy would have preferred it, but the Wright engine was more readily available. The Navy instructed Grumman to build a full-scale wind tunnel model with two different engine nacelles, and finally ordered the prototype with the R-1820 Cyclone. The XF5F-1 had counter-rotating propellers, which would especially improve handling during the take-off and landing phases that are so critical for a carrier-borne aircraft.

Armament choices also created difficulties. The Navy wanted a more effective weapon than the .50 machine gun, but no suitable gun was being manufactured in the USA. A test batch of four Danish 23-mm Madsen cannon had been ordered in 1937, and it was planned to install two of these weapons in the F5F; but this was never done. Ground tests with the Madsen cannon revealed a low rate of fire and poor reliability, and the Navy lost interest. Instead two .50 and two .30 guns were installed in the nose; the centreline installation still offered clear benefits for gun aiming but the aircraft no longer had a firepower advantage over modern single-engined fighters. Perhaps in compensation, the Navy asked Grumman to provide for the carriage of 20 anti-aircraft bombs, internally in the wing. These 2.4 kg weapons were to be dropped on top of an enemy formation, a concept that was widespread in the late 1930s but was sadly deficient in operational value.

The XF5F-1 took to the air on the first of April 1940. Tests were relatively trouble-free, although the unusual configuration caused some aerodynamic problems that resulted in repeated modifications during a long test programme. The new fighter was about 80 km/h faster than the single-engined F4F Wildcat and possessed an impressive rate of climb, which resulted in the type being dubbed the Skyrocket. Nevertheless, the type would never enter production, and this was quite obvious by the end of 1940. Hence, in January 1941, Grumman asked for the termination of the program.

The reality was that, like other small twin-engined fighters, the Skyrocket lacked development potential and a future. The weight growth of the prototype was already a concern before it first flew, and there was little room for the additional equipment needed to upgrade it to a combat type. Contrary to earlier expectations, engines were appearing that had twice the power of the R-1820. The Navy was already awaiting Voughts F4U, which was single-engined, but about as big and heavier than the XF5F-1, and carried heavier armament. Similar to the contemporary Westland Whirlwind, the Skyrocket was a technological dead end, overtaken by the rapid development of the 1940s.

Nevertheless the aircraft kept flying and being modified. As late as November 1942, the XF5F-1 reached its final form, with a longer nose, which again extended over the wing's leading edge to reduce drag and make gun installation easier. It then also featured lengthened engine nacelles, wing root fillets, and propeller spinners. This development work was useful to support the later Grumman F7F.

Lockheed P-38 Lightning

In the 1920s and early 1930s, the US Army Air Corps had conformed to the norms set by other air forces worldwide. It was very conservative in its technical requirements and had no money for new developments. In 1934 the Army did order a monoplane fighter, the Boeing P-26, but this retained as much of the concept of the biplane fighter as a monoplane possibly could: It was small and light, powered by a relatively light engine and weakly armed; it possessed fixed landing gear and wire-braced wings. It was a good dogfighter, but crucially, the P-26 was less modern that the latest bombers, which outpaced it with some ease.

To the more ambitious minds in the Air Corps, the lesson was that the force needed an interceptor, a fast-climbing, high-speed fighter with heavy armament, designed specifically to destroy enemy bombers. Such design goals were common enough in Europe, where most nations had the enemy on their doorstep, but they were less obvious in the USA, if only because nobody could really explain where these enemy bombers were likely to be based. However, this issue did not matter very much, because for the visionaries in the Air Corps, the interceptor specification represented more a way to break with tradition and demand a serious increase in performance, than an actual operational task.

Because both single-engined or twin-engined interceptor designs had their advocates, and general Westover admitted that theoretical studies could not really determine which option was better, specifications were finally issued for both. Specification X-608, issued in February 1937, was for the twin-engined fighter. X-609 was its single-engined equivalent, and would lead to the Bell P-39 Airacobra. The key to the required dramatic performance increase over fighters already in service was the combination of the new Allison V-1710 engine with General Electric turbochargers, as already pioneered by the FM-1. During World War 2, only the USA fielded operational aircraft with turbochargers: The idea was not new, but turning it into a practical device was remarkably difficult, and few succeeded. The turbocharger could give the aircraft equipped with it matchless performance at very high altitude. However, to reach the required 360 mph (580 km/h) at 20,000 ft (6100 m), a much sleeker design than the elephantine FM-1 had to be prepared.

Vultee's design XP 1015 showed what a conventional layout for such an aircraft would be. It featured a long, very slender fuselage with a bubble canopy for the pilot, and a tapering wing. The engine nacelles were highly streamlined, thanks to coolant radiators buried in the outer wing panels, and the turbochargers were located at their rear. With a wing span of 16.46 m, the XP 1015 was a fairly big aircraft. By 1937 standards it was sleek enough, but today, the design looks uninspired, an awkward hybrid of the Bf 110 fuselage with Mosquito wings. (Later, during the Battle of Britain, a Bf 110 fuselage was shipped to Vultee for structural analysis. It would be interesting to know what the conclusion's of Vultee's engineers were.)

Not so the Lockheed offering. Model 22 was the result of a thorough evaluation of a range of radical options by Hall Hibbard and Clarence Johnson. The complex engine installation, comprising the the engine itself, the turbocharger, and the cooling system, required considerable volume in the engine nacelle, which was going to be long if it had to be streamlined. An advantageous way to incorporate such a nacelle into the aircraft was to extend it into a long tail boom. If these tail booms carried the tail surfaces, to the fuselage could be reduced to an abbreviated nacelle in the center of the wing. (The configuration of the S.M.92, with the pilot seated in one of the booms and no central nacelle, was also evaluated, but not adopted.) The design of Lockheed's model 22 was sleek and elegant, and the predicted performance included a phenomenal 417 mph (670 km.h) at 20,000 ft. In June 1937, Lockheed was chosen as the winner of the contract.

On January 27, 1939, the prototype made its first flight. Lockheed's model possessed the grace of a large wasp, all polished flush-riveted metal and sleek, pointed lines. The C-series V-1710 engines used on the XP-38 had a low trust line and an elongated housing for the gearbox, which resulted in more elegant engine cowlings than most production Lightnings would have. As a portent of problems that would follow later, the XP-38 possessed tiny coolant radiators in the aft tail boom, a small oil cooler scoop, and intercoolers buried in the wing leading edge: Designed to cool the V-1710-11/15 engines at their 1000 hp continuous power, and actually not even sufficient for that purpose, this installation was only expected to permit the engines to run at their full 1150 hp for five minutes. The General Electric B-2 superchargers were located overwing, on top of the tail boom, under a streamlined fairing with a nice inlet scoop -- that too, was not retained on production models. The prototype carried no armament or armour. It's life was short: On February 11, after crossing most of the continental USA at an average airspeed of 350 mph, Lt. Kelsey crash-landed the aircraft on a golf course after the engines lost power on landing approach. The cause was never clarified, but later the loss was attributed to carburettor icing. The XP-38 was beyond repair.

There had not been time to seriously test the XP-38, but nevertheless it had demonstrated the great promise of the concept, and the Air Corps ordered 13 service test aircraft, designated Lockheed model 122, and officially YP-38. However, serious redesign was necessary to turn the XP-38 into a viable combat aircraft, and the YP-38s had few parts in common with their precursor. They featured F-series V-1710 engines with a higher trustline and different gearbox, an entirely redesigned engine installation, a redesigned structure that was a bit lighter and more suitable for mass production, and provisions for armament (which was not installed). This delayed the first flight of a YP-38 to 17 September 1940. The YP-38, a fast aircraft that easily picked up speed in a dive, soon confronted its test crews with the dangerous phenomenon known as at the time as compressibility. The fighter's relatively low critical Mach number of 0.68 was easily exceeded in a power dive. The the accelerated, supersonic air flow over parts over the aircraft there made the control surfaces ineffective. Especially the turbulent wake of the center section, made the elevators ineffective and generated strong vibration in the tail section, so that recovery from such a dive was no longer in the pilot's control. On 5 November 1941, test pilot Ralph Virden lost his life when his aircraft did not survive a test dive. Wind tunnel research led to the introduction of a small fillet at wing root, which at least removed the extreme tail buffeting that had plagued the YP-38s in a high-speed dive, but did not restore controllability at elevated much Numbers. For much of its operational career, P-38 pilots had to refrain from following enemy fighters in a dive.

The YP-38s were not combat-ready fighters, nor were the first P-38s off the production line: All lacked armour and self-sealing fuel tanks. Their elegantly curved, unarmoured windscreens were a clear hint. In August 1941, delivery of the P-38D began, incorporating some of the lessons of combat in Europe, but still with interim armament installation. The army (somewhat optimistically) had decided to give 'D' designations to all fighters that had been brought up to the latest combat standard. The 1st Pursuit Group, based on Selfridge Field in Michigan, was the first to be equipped with these fighters, and in the autumn of 1941 it participated in exercises. At the time of Pearl Harbour, it would be the only fighter group fully equipped with the P-38.

Finding an optimal armament installation was difficult. The original concept combined the 37-mm T9 (in its developed form, M4) cannon, two .50 machine guns, and two .30 machine guns. This was less than ideal. In theory, an excellent armament installation that was possible on a twin-engined fighter, because the trajectory of nose guns did not need to converge to the center; but these guns were a poor match for trajectory. The M4 was a slow-firing weapon with modest ballistic performance, and was in short supply. (Contrary to some reports, the more performant but far heavier M9 cannon never seems to have been installed in the P-38.) In August 1940, the Army decided to install a single 20-mm Hispano cannon and four .50 Browning M2 machine guns, a much combination that was in much better harmony. And while the American-built version of the French Hispano HS.404 cannon had an indifferent reputation at best, it seems to have been relatively trouble-free in the P-38, perhaps because it was firmly supported by its fuselage mounting.

With this armament installation, the P-38E was the first major production version, although the Army still considered it an interim type. Deliveries began in November 1941, but most of the 210 produced were used for training purposes, designed RP-38E with the R of 'restricted', because they were still not considered entirely combat-ready. A few were sent north in 1942, in response to Japanese attacks on the Aleutian islands. The P-38F model, first delivered in March 1942, had 1325 hp V-1710-49/53 engines and the capability to carry external drop tanks or bombs. The P-38F was the first Lightning to be built in significant quantities and sent out to combat theatres. It was followed by the P-38G, with V-1710-51/55 engines and numerous small modifications.

From January 1939 to March 1942, more than three years had passed from the first flight of a prototype to the introduction of a model that was considered entirely fit for combat. (And which, incidentally, was still having some problems with its engine installation, specifically the intercoolers.) This gestation period was considerably longer, for example, than the development and production run of the Westland Whirlwind. The USAAF's perseverance with the P-38 was a sign of their great confidence in the future of the type, and perhaps also of their lack of good alternatives. At the same time it was a strong indication of the risks that Lockheed had taken with the ground-breaking design of the P-38.

Of course, meanwhile there had been another model of the fighter Lightning, the Lightning Mk.I ordered by and built for the RAF — a type not unfrequently referred to as the castrated Lightning, because it was powered by C-series Allison engines, without the turbochargers and handed propellers. It has to be taken into account, however, that the British took the bold step to order a large numner of Lockheed model 322-B Lightnings in March 1940, well before the first YP-38 had flown. (And the French ordered even more.) If foregoing the most advanced features of this new fighter was an error, it was at least erring on the side of caution, and not unreasonable considering that the buyers were allocating virtually unprecedented amounts of cash in the hope of taking delivery of advanced fighters within one year. And an even larger order for Lightning Mk.II fighters with turbosuperchargers followed right on the heels of that contract, showing that the RAF was well aware of the value of his equipment. But when the first Lockheed model 322-61 arrived in England in December 1941, both the strategic and the tactical situation had changed in the wake of the Battle of Britain. The RAF could now afford to choose, and it chose not to have the Lightning, which in its Mk.I form and at the time of testing was handicapped by poor high-altitude performance, speed restrictions because of the unresolved tail vibration problems, and relatively poor manoeuverability for a fighter. Perhaps crucially, Britain had already received its first Mustang Mk.I in October. The Mustang Mk.I had the same restrictions on high-altitude performance as the Lightning Mk.I, but it was much more adequate in most other respects.

In July 1942, the first P-38F fighters of the Eight Air Force arrived in Britain, having flown across the Atlantic as part of operation Bolero. Ready for operations in August, they stayed in Britain only a short time before being moved to the Meditteranean Theatre of Operations (MTO) in support of operation Torch. In the MTO the Lightning proved valuable because of its range and firepower, but combat experience was not entirely satisfactory. The P-38 proved vulnerable in combat with German fighters, at least in part because of tactical errors and the tendency of pilots to engage in turning combat instead of exploiting the excellent zoom climb capability of their aircraft. The P-38 could make very tight turns, but its size and mass distribution resulted in a disadvantage in roll rate, so it could not enter turns quickly. Because of its size and unique configuration, the P-38 was also easily seen and identified in the air, which put it at a tactical disadvantage. In late 1942 the P-38s also appeared in the South-West Pacific Area (SWPA) as part of the 5th Air Force, flying its first combat missions there in December. Here too the excellent range of the fighter was of great value, and tactical problems were smaller. It had already been recognied that it was foolish to attempt to 'mix it' with the nimble Japanese fighters, but on the other hand the A6M or Ki-43 could not match the speed of the Lightning.

The scale of P-38 operations soon proved difficult to maintain, with losses exceeding the production rate, and commanders in the South-West Pacific were told that the MTO had priority. By September 1943, the Pacific had 212 operational P-38s and the Mediterranean fewer than 200. New aircraft were destined for the European Theatre of Operations (ETO), were a long-range bomber escort force was urgently needed. In November 1943, P-38s started flying operations against targets in Germany.

By that time, a new Lightning was making its debut. Allison had created the V-1710-F17 engine — left and right hand turning engines as installed in the P-38 were known as the V-1710-89/91 by the military — which thanks to higher supercharger gearing and 100/130 octane fuel, delivered more power. They were rated for 1425 hp with a War Emergency Rating (WER) of 1600 hp. However, the existing engine installation of the P-38 did not permit such power to be actually used, becuase of the too small intercooler capacity. So although the P-38H had the -89/91 engines, it could not use their full power. For this more redesign was necessary, including the introduction of a core-type intercooler located in a chin intake under the propeller hubs. The result was the P-38J, which entered production in August 1943. At the same time production finally came up to speed, to that in the end three-quarters of all P-38s featured the new engine nacelle.

But operating at high altitudes over Europe to escort heavy bombers, the new engines failed. Often catastrophically so, with bearing failing and connecting rods being thrown. The spate of failures and crashes seriously undermined confidence in the P-38J, by early 1944, attempts to operate in cold weather resulted in a quarter or even half of the fighters turning back with engine problems. Flying at lower altitude in warmer air reduced the number of failures, but at the cost of putting the P-38 at a tactical disadvantage.

Because it was a twin-engined fighter, it seemed logical to assume that a P-38 would be able to safely return to base if one engine failed or was damaged by enemy fire. But reality, of course, was that the odds for survival were not good if over enemy territory, for the losses of stragglers that fell out of formation were always very high. A hit in the engine also frequently lead to the damaged engine catching fire. In this respect the streamlined engine installation of the P-38 was a serious disadvantage, in that the engine installation was a dense but long package, and very vulnerable to enemy fire.

There may not have been a single cause. Probably poor pilot training, too much use of the 1600 hp WER without corresponding maintenance, and fuel quality problems all played a role. A weakness was found in the engine: The new intercoolers were a bit too effective, reduced temperature in the carburetor inlets too much, and thereby caused condensation of the fuel and the the tetra-ethyl-lead additive. Modifications by Lockheed and Allison cured that problem, but that took months, and for the P-38 the time lost was crucial. From June 1944 onwards, P-38 Fighter Groups in the 8th started to be converted onto the P-51.

Ironically, this decision was made at a time when the P-38 finally reached its fully mature form. Dive flaps, installed from the P-38-J-25 onwards, finally made high-speed dives safe. Power-boosted ailerons reduced control forces and transformed the combat handling of the P-38, giving it a roll rate better than that of many single-engined fighters. The V-1710-111/113 engine featured many improvements, and could deliver 17285 hp WER on 150 octane fuel. From June 1944 onwards, the latest P-38s could be said to the equal or the better of the best single-engined fighters in air combat; a great achievement for a large twin-engined fighter. This was something the RAF pilots who flew the Lightning Mk.I in December 1941 and rejected it, probably could not have guessed. Of course, June 1944 was quite late in the war. This permits some of the controversy surrounding the Lightning to persist, forever. Its defenders can point to the P-38L of 1944, definitely one of the best fighters of the war. Its detractors can comment on the P-38F of 1942, struggling with the Bf 109 and Fw 190 and generally emerging second best.

Anyway, the P-38 was no going to catch back the limelight as the 8th AF's escort fighter, but it was increasingly doing at least equally valuable and much more dangerous work in the 9th AF.

Lockheed P-49

Grumman P-50

Lockheed P-58 Chain Lightning

Douglas DB-7 Havoc and P-70

Northrop P-61 Black Widow

McDonnell XP-67

Nachtjagd

Dornier Do 17Z Kauz

Dornier Do 215B-5 Kauz III

Dornier Do 217

Messerschmitt Bf 110

Messerschmitt Me 210 and 410

The Arado Ar 240, as it was flown in April 1940, had a conventional layout but nevertheless some radical characteristics. Wing loading was very high for the period, with large Fowler flaps and automatic leading-edge slats to get acceptable landing characteristics. To reduce drag, the engine nacelles and fuselage were slender, but long, and annular radiators were installed on the engine nacelles, with large hollow propeller spinners. The rear was defended by an upper and a lower FA 13 turret, each containing two MG 81 machine guns. These were remotely controlled by the gunner, who aimed with two remote periscopic sighting heads. The Ar 240 was fast, and for this reason it saw limited use as a reconnaissance aircraft. But its handling qualities were bad, even dangerous, and made it unsuitable as a fighter.

Junkers Ju 88

Focke-Wulf Ta 154

Heinkel He 219

Junkers Ju 388

The Last Twins

Dornier Do 335

Tupolev Tu-1

de Havilland Hornet

Grumman F7F Tigercat

Douglas XA-26A

North American F-82 Twin Mustang

FMA I.Aé.30 Ñamcú

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