The D.520 was designed in response to a 1936 requirement from the French Air Force for a fast, modern fighter with a good climbing speed and an armament centred on a 20 mm cannon.

Unlike the Morane-Saulnier M.S.406, which was at that time the most numerous fighter in the French Air Force, the Dewoitine D.520 came close to being a match for the latest German types, such as the Messerschmitt Bf 109.

[2] The design had been rejected by the French Air Ministry, which, after being impressed by the British Hawker Hurricane and Supermarine Spitfire, had decided to respond by uprating the specifications to include a maximum speed requirement of 500 km/h (310 mph).

[6] In addition to the lack of a prototype order, Dewoitine was absorbed into the larger Société nationale des constructions aéronautiques du Midi (SNCAM) state-owned manufacturing consortium.

[5] On 2 October 1938, the first prototype aircraft, D.520-01, powered by a Hispano-Suiza 12Y-21 engine that temporarily drove a fixed-pitch, two-bladed wooden propeller, performed its maiden flight.

[5] These included changing the engine to a newer -29 model and incorporating exhaust ejectors which provided added thrust, along with a three-blade variable-pitch propeller.

Overall, flight tests had proceeded successfully, and resulted in the issuing of a contract in March 1939 for 200 production machines to be powered by the newer -31 engine (later replaced by the -45).

[17] As the first batch of D.520s rolled off the production line, they failed acceptance tests due to insufficient top speed and troublesome cooling.

[18] Redesigned compressor intakes, a modified cooling circuit and propulsive exhaust pipes proved to be effective remedies of these shortcomings, but as early examples had to be retrofitted with these improvements, the type was not declared combat ready until April 1940, at which point the D.520's operational standards had been defined.

[19] In order to speed up the redesign process, a total of four production aircraft were handed back to SNCAM to serve as special test machines.

The French Air Ministry planned for over 300 aircraft a month to be built and managed to reach this goal, especially in June 1940, but it was too late to affect the tide of battle.

[2] During 1940, negotiations with American manufacturer Ford had been underway with the aim of establishing the licensed production of an Americanised version of the type, designated as the D.522.

The intention was for the type to replace all other single-engine fighters that remained in service with the Vichy French Air Force and to eventually equip additional units that were to be reformed from September 1942 onwards.

56 carburettors mounted on an inlet manifold which directed compressed air from the supercharger to the engine cylinders; the 12Y-45 and -49s fitted to production D.520s used either 92 or 100 octane fuel.

351 were fitted with the 12Y-49 960 CV (710 kW; 950 hp) engine driving a Chauvière type 3918 pneumatically operated propeller, 3 m (9.8 ft) in diameter.

[29] Equipment included a Radio Industrie Type Rl 537 radio-receiver set, an OPL RX 39 reflector sight (less effective than the Revi system), a height-adjustable seat, and a sliding canopy with large, clear panels.

[18] In April and May 1940, operational units received 34 'war-capable' production D.520s; the type quickly proved to be highly popular with pilots and ground crew.

During comparative trials on 21 April 1940 at CEMA at Orleans-Bricy against a captured Bf 109E-3, the German aircraft had a 32 km/h (20 mph) speed advantage owing to its more powerful engine.

[2] By 10 May 1940, when the Phoney War came to an end as Germany launched the invasion of France and the Low Countries, a total of 246 D.520s had been manufactured, but the French Air Force had accepted only 79 of these, as most others had been sent back to the factory to be retrofitted to the new standard.

[35] One of the most successful D.520 pilots was Pierre Le Gloan, who shot 18 aircraft down (four Germans, seven Italian and seven British), scoring all but two of his kills with the D.520, and ranked as the fourth-highest French ace of the war.

The Vichy Air force was numerically strong, but with very few ground crew and spare parts, which meant that the operational flying time for the D.520s was very limited.

[40] Over the following days several escort missions were flown to protect Martin, LeO and Bloch 200 (3/39 Esc) bombers from British Royal Navy fighters.

On 26 June, a strafing run by Tomahawks of 3 Sqn, on Homs airfield, destroyed five D.520s of Fighter Squadron II/3 (Groupe de Chasse II/3) and damaged six more.

From early 1943 on, they were relegated to training duties at the fighter school in Meknes, and progressively replaced by Supermarine Spitfires and Bell P-39 Airacobras in combat units.

Commanded by former test pilot Marcel Doret, one such unit attacked German forces at Royan and Pointe de Grave, performing strafing runs upon enemy artillery positions, as well as providing air cover for Allied bombers.

[45] After 1 December 1944, the date on which the French Air Force was officially reformed, Doret's unit became G.C.II/18; it continued to operate D.520s for several months before being reequipped with Spitfires on 1 March 1945.

On 21 May 1943, the Regia Aeronautica and the Luftwaffe agreed to exchange 39 Lioré et Olivier LeO 451s, captured by the Italians at the SNCASE factory in Ambérieu-en-Bugey (Lyon), with a stock of 30 D.520s.

[51][52] The captured Dewoitines were to be delivered to the Axis Balkan Front, although some were used by the Luftwaffe for training purposes while 60 were transferred to Italy and 96,[53] or 120, to the Bulgarian Air Force[54] for use in combat.

To intercept the 450 bombers (B-17 Flying Fortresses, B-24s and Handley Page Halifaxes) escorted by 150 P-38s, the Bulgarians scrambled 28 Dewoitines from I./6th at Karlovo, six D.520s from II/6th at Vrazhdebna (together with 39 Bf 109G-6s and even Avia 135s).

That day the Bulgarian Air Force suffered the heaviest losses since the beginning of the war: nine fighters shot down and three that had to crash land.