In the mid-1930s, Czechoslovakia sought to modernise its air force, leading Avia to design the B.35 monoplane fighter as a successor to the B-534 biplane.
The B.35 blended modern ideas with older features, using a steel tube fuselage with metal skin at the front, fabric at the rear, wooden elliptical wings, and a fixed, spatted undercarriage, powered by an 860 hp Hispano-Suiza V-12.
Despite some outdated aspects, it proved highly manoeuvrable, and Avia evolved it into the B.35/3 prototype with retractable landing gear, all-metal wings, and better aerodynamics and visibility.
This became the basis for the B.135, finalised in 1938, featuring a cantilever low-wing design, an enclosed cockpit with a raised spine, and armament of a 20 mm MG FF cannon and two 7.92 mm vz. 30 machine guns, while retaining the Hispano-Suiza engine and fixed-pitch wooden propeller.
Bulgarian officers were impressed, ordering 12 aircraft built in Czechoslovakia, along with engines and a licence for local production as the DAR 11 “Lyastovitsa”, though only the original 12 were made before German authorities halted production.
Engine reliability issues limited the B.135 mostly to training roles, though on March 30, 1944, four of them intercepted USAAF bombers over Bulgaria, possibly downing a B-24 in one of their few combat actions.
