In July 1934 the French air ministry issued a requirement for a modern single-seat fighter to replace types such as the Dewoitine D.510, which had been rendered obsolete by the advent of the first ‘modern’ fighters such as the Hawker Hurricane with features such as a low-set cantilever wing (also possessed by the D.510) carrying flaps on its trailing edges, an enclosed cockpit, landing gear with retractable main units, and a powerplant capable of delivering about 1,000 hp (746 kW) and of being adapted to drive a three-blade propeller of the variable-pitch type. The first effort by the Société des Avions Dewoitine to meet this requirement was the D.513 which first flew in prototype form during January 1936 with a powerplant of one Hispano-Suiza 12Ycrs-1 engine with a circular frontal radiator located mainly under the propeller hub. The prototype had an enclosed cockpit and inward-retracting main landing gear units, but the wing was too large (elliptical outer panels attached to a broad constant-chord centre section) and, with a maximum speed of only 229 kt (264 mph; 425 km/h) at 15,090 ft (4600 m), its performance was distinctly disappointing. The aeroplane was therefore taken in hand for very extensive reconstruction in which only the wing and tailplane remained unaltered for marriage to a new fuselage, vertical tail surface, main landing gear units, and engine installation. This last was based on the Hispano-Suiza 12Ydrs-2 engine rated at 930 hp (693 kW) at 3,250 ft (990 m) and cooled by water passing through a radiator installed in a deep bath under the nose. In this revised form the D.513 could attain a maximum speed of only 240 kt (276 mph; 376 km/h) at 16,730 ft (5100 m), and was also beset by continued instability in the longitudinal and directional planes as a result of the short rear fuselage, engine cooling problems and landing gear retraction difficulties.
Thus the Morane-Saulnier MS.405 was preferred to the D.513 for the resulting production order, and Dewoitine therefore decided to start afresh in his effort to produce a ‘modern’ fighter for the French air force. The first result of this decision was the creation of an independent design office, under the supervision of Robert Castello with Jacques Henrat as his main assistant, for the design of a fighter to be capable of a maximum speed of 270 kt (311 mph; 500 km/h) on the 900 hp (671 kW) provided by a Hispano-Suiza 12Y-21 engine. Dewoitine submitted the team’s first design to the French air force, which rejected the concept as it had set its sights on a new fighter with a maximum speed of 281 kt (323 mph; 520 km/h). Castello therefore revised the initial design with a wing of reduced span and a new Hispano-Suiza engine promising 1,200 hp (895 kW), and this received the designation D.520 in recognition of the air force’s speed requirement. The D.520 was officially rejected in favour of the Morane-Saulnier MS.406 when submitted in response to a C.1 (Chasse monoplace, or single-seat fighter) requirement issued in June 1936, but the company decided to press ahead with the detail design and the construction of two prototypes as a private venture in the hope that the new fighter would finally gain official approval. This was finally forthcoming only as the first of the prototypes was nearing completion, and it was at this juncture in April 1938 that the two prototypes received an official contract that also stipulated the construction of a third prototype.
The D.520 was of classic ‘modern’ fighter design, being a cantilever low-wing monoplane with an all-metal structure of the stressed-skin type. In its definitive form, the airframe was based on the oval-section fuselage which accommodated the engine in the nose, the main fuel tank on the centre of gravity, and the pilot’s enclosed cockpit (with a rearward-sliding section for entry and exit) just above the trailing edge of the wing roots. This structure also supported the flying surfaces, which comprised a cantilever tail unit with a high-set tailplane, and a wing that was set at a comparatively large dihedral angle, was tapered in thickness and chord, carried over virtually the full span of its trailing edges the now-standard combination of outboard ailerons and inboard flaps, and accommodated two additional fuel tanks to supplement the fuselage-mounted tank’s supply on longer-range sorties. The Hispano-Suiza 12Y engine was cooled by ethylene glycol liquid passing through a radiator installed in a ventral bath below the wing, and was fitted with a moteur-canon installation for a 20 mm cannon (with 60 rounds of ammunition) firing forward through the hub of the three-blade Ratier metal propeller of the variable-pitch type, and the rest of the armament comprised four 0.295 in (7.5 mm) forward-firing machine guns located in the wing leading edges immediately inboard of the auxiliary fuel tanks. The airframe was completed by the tailskid (soon changed to tailwheel) landing gear whose inward-retracting main units were located in the underside of the wings just inboard of the two machine gun installations.
The D.520.01 first prototype recorded its maiden flight on 2 October 1938 with the powerplant of one Hispano-Suiza 12Y-21 engine driving a two-blade wooden propeller of the fixed-pitch type, and differed from the later standard in other details such as its lack of armament and use of two underwing radiators for engine cooling. In this form the prototype was a disappointment as it attained a maximum speed of only 262 kt (301 mph; 485 km/h), but matters improved considerably after the machine was modified with a Hispano-Suiza 12Y-29 engine driving a three-blade propeller of the variable-pitch type and the two underwing radiators were replaced by a single unit in a ventral bath: in this form the first prototype attained its designed speed in level flight, and proved itself to possess a high diving speed.
The D.520.02 second prototype took to the air for the first time on 28 January 1939, and differed from the first prototype in its full armament, strengthened landing gear, revised cockpit, and redesigned tail unit. The second prototype attained a maximum speed of 284 kt (327 mph; 527 km/h), and was followed by the D.520.03 third prototype, first flown on 5 May 1939, with a Szydlowski supercharger in place of the Hispano-Suiza unit fitted on the first two prototypes.
At the outbreak of World War II in September 1939, only these three prototypes had flown. This was a sad reflection on the stage of the French air force and French aero industry at this time: at the end of 1938, the French air force could field some 380 single-engined fighters (only 16 of them ‘modern’ MS.405 and MS.406 aircraft) and the position of the Royal Air Force was not drastically dissimilar; yet by the start of World War II the RAF had 485 ‘modern’ Hurricane and Supermarine Spitfire fighters whereas the French air force had only 270 ‘modern’ MS.406 fighters that were, as events were soon to prove, wholly inferior to the Messerschmitt Bf 109E fighter that the Germans flew against them. The problems for France were that too much effort was being devoted to the development and evaluation of a host of prototypes rather than to the production of warplanes that early trials had proved adequate, and that the recently nationalised aero industry, in which Dewoitine had been transformed into the Société Nationale de Constructions Aéronautiques du Midi, was undergoing a reorganisation that seriously delayed production of most types.
Production and service
Early in 1939, however, the French air force realised the parlous nature of its fighter capability and, among other orders, contracted in April 1939 for the delivery by the end of the year of an initial batch of 200 D.520C.1 fighters based on the third prototype with a powerplant of one Hispano-Suiza 12Y-45 engine fitted with the Szydlowski-Planiol supercharger. The deepening political crisis that was soon to plunge Europe into the first campaigns of World War II was recognised by the placement of a second order in July 1939 for an additional 510 D.520 fighters, and further contracts were signed in September 1939, January 1940, April 1940 and May 1940 for further deliveries that would raise the total of D.520 fighters to 2,200 aircraft including 120 for the French naval air arm produced at a rate that was to increase from 50 aircraft per month in November 1939 to 350 aircraft per month in November 1940.
In the event this ambitious programme was overtaken by events, which culminated in France’s surrender to Germany in June 1940, just over six weeks after the start of the German offensive that also drove Belgium and the Netherlands into defeat and expelled the last British forces from the continent. The overambitious nature of French plans for D.520 production was revealed by the delivery of the first production aeroplane only in November 1939 and of only another 12 machines by the first day of 1940. Even these aircraft were not fully operational, for they had a maximum speed of merely 280 kt (316 mph; 509 km/h) as they had only an interim supercharger inlet and lacked their planned ejector exhaust stubs. Thereafter the delivery rate improved, and by the time of the German onslaught Groupe de Chasse I/3 had 36 operational fighters, and non-operational aircraft had been delivered to GC I/3, III/3 and II/7 for use as conversion trainers. Additional aircraft were delivered as the German campaign forged into France, and by the time of the French surrender some 437 aircraft had been completed and 351 delivered for use by a total of five groupes de chasse, whose pilots claimed 108 confirmed and 30 probable ‘kills’ against German and Italian warplanes for the loss of 85 aircraft, only 54 of them to direct enemy action, on operations and a further 21 in training accidents.
Another 52 aircraft had been accepted by the French naval air arm, but this service’s Escadrilles AC1, AC2, AC3 and AC4 failed to get into action before the French surrender. Of the surviving aircraft, 153 were located in the part of France unoccupied by the Germans (soon to become Vichy France), 175 were flown to North Africa, and three escaped to England.
The Germans initially banned the use of the D.520 by Vichy French units in metropolitan France, and the D.520 was thus used by fighter squadrons based in North Africa and Syria: in the latter theatre the D.520 was used against the British during the campaign of 1941, claiming 31 ‘kills’ for the loss of 32 of their own number. In April 1941, however, the Germans agreed that the D.520 could be deployed in metropolitan France and agreed that another 1,074 aircraft of this type could be produced in Vichy French factories, most notably the Toulouse facility of the Société Nationale de Constructions Aéronautiques du Sud-Est that had taken over from the defunct SNCAM. An initial order was placed for 550 aircraft and 349 of these had been delivered by November 1942, when German forces overran Vichy France in response to the Anglo-American landing of that month in French North-West Africa. The last 197 of these new aircraft were fitted with the Hispano-Suiza 12Y-49 engine with a higher rated altitude and driving a Chauvière propeller for better performance at high altitude.
In three days of fighting before the French North-West African territories switched sides to the Allied camp, D.520 fighters saw some action against Allied aircraft, losing 32 of their own number (19 naval air arm and 13 air force aircraft). When they seized the unoccupied part of France in November 1942, the German forces captured 1,876 aircraft including 246 operational D.520 fighters. Another 169 D.520 fighters were seized at the Toulouse factory: 19 of these were in flying condition, and in March 1943 the Germans ordered the 150 incomplete aircraft to be finished in a programme that saw the delivery of the last D.520 in just over 12 months. This raised the number of D.520 fighters built to 905. Many of these aircraft were used by the Luftwaffe as fighter trainers with three Jagdgeschwadern (fighter groups) based in France (two) and Austria (one), but later the type was flown operationally on the Eastern Front against the Soviet air forces. Some 60 D.520 fighters were passed to the Italian air force for service with four squadrons in the home defence role, a few aircraft were allocated to the Romanian air force but not delivered, and 120 D.520 fighters were transferred to the Bulgarian air force. Finally, a number of aircraft returned to French service after the Allied liberation of France after June 1944. Two escadrilles saw service against the Germans, but the D.520 was used mostly for training and remained operational in this role until 1953, by which time some 13 of the aircraft had been converted as D.520DC (Double Commande, or dual-control) two-seaters.
Variants of the D.520 that remained either as projects or prototypes included the D.520Z conversion of a D.520 with a different cooling system and Messier landing gear, the SE.520Z prototype development of the D.520 by Sud-Est with heavier armament and the Hispano-Suiza 12Z engine, the D.521 prototype with the powerplant of one Rolls-Royce Merlin III liquid-cooled V-12 engine rated at 1,030 hp (768 kW) and provision for a fixed forward-firing armament of two 20 mm HS-404 cannon and two 0.295 in (7.5 mm) MAC 34 M39 machine guns, the D.523 prototype conversion of a D.520 with the Hispano-Suiza 12Y-51 engine, the D.524 conversion of the D.521 with the Hispano-Suiza 12Z-89ter engine, the D.550 racer project with the Hispano-Suiza 12Ycrs (later Hispano-Suiza 12Y-51) engine, the D.551 fighter counterpart of the D.550 of which 18 were built but never flown, and the HD.780 conversion of a D.520 as prototype for a twin-float fighter model.
Type: interceptor fighter
Accommodation: pilot in the enclosed cockpit
Armament: one 20 mm Hispano-Suiza HS-404 fixed forward-firing cannon with 60 rounds in a moteur-canon installation to fire through the hollow propeller shaft, and four 0.295 in (7.5 mm) MAC 1934 M39 fixed forward-firing machine guns with 675 rounds per gun in the leading edges of the wing
Equipment: standard communication and navigation equipment, plus an OPL RX 39 reflector gun sight
Powerplant: one Hispano-Suiza 12Y-45 liquid-cooled V-12 piston engine rated at 930 hp (693 kW) for take-off and 920 hp (656 kW) at rated altitude or, from the 559th aeroplane, one Hispano-Suiza 12Y-49 liquid-cooled V-12 piston engine rated at 900 hp (671 kW) at take-off and 910 hp (678.5 kW) at rated altitude
Fuel: internal fuel 246.4 Imp gal (295.9 US gal; 1120 litres); external fuel none
Dimensions: span 33 ft 5.5 in (10.20 m); area 171.69 sq ft (15.95 m²); length 28 ft 8.75 in (8.76 m); height 8 ft 5.25 in (2.57 m) with the tail down; wheel track 9 ft 5.5 in (2.88 m)
Weights: empty 4,685 lb (2125 kg); normal take-off 5,897 lb (2675 kg); maximum take-off 6,151 lb (2790 kg)
Performance: maximum level speed ‘clean’ 291 kt (336 mph; 540 km/h) at 23,030 ft (7020 m) declining to 224 kt (258 mph; 415 km/h) at sea level; cruising speed, economical 200 kt (230 mph; 370 km/h) at optimum altitude; initial climb rate 2,815 ft (858 m) per minute; climb to 13,125 ft (4000 m) in 5 minutes 49 seconds; service ceiling 36,090 ft (11000 m); maximum range 831 nm (957 miles; 1540 km) with wing tanks full; typical range 480 nm (553 miles; 890 km) with wing tanks empty
Operators: Bulgaria (120), France (736), Germany (not available) and Italy (60)