Issue no. 148 of the 'la Vie Aérienne' dated 22 March 1939 - front page coverage of the prototype D.520, the "fastest French fighter ever conceived" powered by an Hispano Suiza engine. A shot from a test flight of prototype n°1 flown during January 1939 with Constantin Rozanoff at the controls..
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. Design work on the D.520 started in September 1936 led by Émile Dewoitine. Dewoitine had been disappointed with the performance of his last design, the Dewoitine D.513, which was rejected by the French Air Force in favour of the Morane-Saulnier M.S.406, and decided to respond to the specifications with a design using the latest construction techniques and the most powerful available engine, the new 660 kW (890 hp) Hispano-Suiza 12Y-21 liquid-cooled engine. The nationalisations undertaken by the Popular Front government during 1937 caused a seven-month delay and interruption in the design work that according to French author Chris Ehrengardt was never caught up as Dewoitine's Toulouse factories were amalgamated into a large conglomerate, the SNCAM. (Construction Aeronautique du Midi) In the meantime the French Air Ministry, impressed by the British Hawker Hurricane and Supermarine Spitfire, had uprated the specifications to include a maximum speed requirement of 520 km/h (310 mph). In response, Dewoitine renamed the further development, the "D.520". The D.520-01, powered by a Hispano-Suiza 12Y-21 temporarily driving a fixed-pitch, two-bladed wooden propeller, first flew on 2 October 1938, but managed to reach only 480 km/h (300 mph) in flight tests, and suffered from dangerously high engine temperatures. Most of the problem was judged to come from greater than expected drag from the under wing radiators, which exhausted across the upper wing surface, and these were replaced with a single radiator unit housed under the fuselage in a streamlined fairing. Tail fin and rudder were also enlarged. After minor damage in a landing accident, further modifications included changing the engine to a newer -29 model and incorporating exhaust ejectors for added thrust (so-called 'pipes a reaction'), along with a three-blade variable-pitch propeller. These changes were enough to allow the aircraft to reach its design speed, D.520-01 achieving 530 km/h (330 mph) on 13 January 1939.
Below; 11 April 1940 issue of 'Flight' courtesy of the Flight Global magazine archive