The first prototype, the B-35/1 displayed excellent flying characteristics and high speed and was originally powered by a Hispano-Suiza 12Ydrs piston engine. The powerplant was later changed to a 12Ycrs with an identical power output, but with provision for an autocannon to be fitted between the cylinder banks to fire through the propeller hub. Testing continued until 22 November 1938, when the aircraft was destroyed in a crash that killed Avia test pilot Arnošt Kavalec. Nevertheless, a second prototype, B-35/2, was already reaching completion, and was fitted with redesigned ailerons and flaps. It first flew on 30 December with testing beginning in earnest in February 1939. A pre-production series of ten aircraft was ordered, but before these could be built, Czechoslovakia was occupied by Germany in March 1939.
Development however was resumed under German control, with the substantially revised B.35/3 flying in August 1939. The elliptical leading edges of the wings had been replaced with straight ones, and an outwardly-retracting main undercarriage was fitted. This prototype was the first to actually carry its intended armament. Now, wearing German markings and the registration D-IPBB, it was displayed at the Salon de l'Aéronautique in Brussels, where enough interest was generated to spur development of an improved version of the aircraft as the B.135.