A new type was needed to bridge the gap between the L-29 and the more modern jet fighters of the day. As the Soviet Air Force, Aero’s largest customer at the time, was looking for just such an aircraft for advanced jet trainer, it once again joined forces with the VZLÚ (Aeronautical Research and Test Establishment) to propose a solution. In addition, during February 1964, the Czech Ministry of Defence also issued a preliminary technical specification setting out the requirements for developing a new jet trainer.
While development continued and series production started, in late 1971 the training of Czech Air Force instructors on the new type began at the Vodochody factory. An initial five pre-production airframes, still fitted with the initial AI-25W powerplant, were handed over on March 28, 1972, to the Military Aviation School at Košice. This enabled development of new pilot training programmes for the aircraft, which was planned to act as a more advanced follow-on from basic training in the L-29 Delfin. As it turned out, the excellent handling qualities of the aircraft in all training regimes meant that it could also perform the basic role. It would eventually also replace the L-29, cutting costs by making it necessary to provide maintenance and support for only a single aircraft type instead of two. By this time the name Albatros had been adopted for the L-39.