First flown on 15 April 1935, the D 503 was, in fact, a modification of the D 511 prototype, which, although exhibited in the Salon de l'Aeronautique in Paris in November 1934, was not flown. The D 511 had consisted of the fuselage and tail assembly of the D 500 mated to a wing of marginally smaller span and area, cantilever main undercarriage members and an Hispano-Suiza 12Ycrs engine. Calculations indicated that the D 511 would not offer a sufficient advance in performance (despite the fact that the first production order for the D 510 was not to be placed until May 1935) and it was therefore re-engined with a 690hp HS 12Xcrs engine with a circular frontal radiator, armament comprising a 20mm engine-mounted cannon and two wing-mounted 7.5mm Dame machine guns. Flight testing revealed a climb rate inferior to that of the standard D 500 and, after serving as the personal aircraft of Col Rene Fonck in the escadrille ministerielle, the prototype was assigned to the flying school at Etampes.