How did the Blériot-SPAD S.510 win a contract despite losing the competition?

The program required 325 km/h (202 mph; 175 kn) at 3,500 m (11,500 ft), a figure later raised to 350 km/h (220 mph; 190 kn) at the same altitude, with an armament of four machine guns, or one 20 mm cannon and two machine guns, or two cannons. Herbemont started from his earlier S.91, and of the eleven projects submitted to meet the requirement, his was the only biplane. The aircraft was of all-metal construction, with fabric-covered wings and tail and a monocoque rear fuselage.

In the 1935 evaluation at the Centre d'expériences aériennes militaires (CEAM), the French military aviation research centre at Reims, the S.510 was compared against the Dewoitine D.500, which was already in production. There it demonstrated better manoeuvrability and a better rate of climb.

The S.510 was doomed to obsolescence even before it flew, although when it was designed many pilots and experts firmly believed that biplanes would prove better fighters than monoplanes due to their tighter turning radius. Despite its merits, it enjoyed only about a year of usefulness: a fighter suited to 1936, it was soon overtaken by the newer, more modern monoplanes developed by Germany, Britain and France.