In the late 1950s, the Soviet Air Force was seeking a jet-powered replacement for its fleet of piston-engined trainers, and this requirement was soon broadened to finding a trainer aircraft that could be adopted in common by Eastern Bloc air forces. Aero's response, the prototype XL-29 designed by Z. Rublič and K. Tomáš first flew on 5 April 1959, powered by a British Bristol Siddeley Viper engine. The second prototype was powered by the Czech-designed M701 engine, which was used in all subsequent aircraft.