In 1946 (he first Soviet jet fighters successfully passed State tests and were adopted by Air Forces. Practically at once the need for corresponding planes with double-manning appeared. The serial fighter Yak-15 was taken as a base of the construction of training aircraft Yak-21 in Yakovlev design office. The armament was taken away and on its place in front of pilot`s cabin, designers placed the second cabin for the trainee.
Simultaneously with Yak-21, with regard for heightened demands to lighter, the staff of Yakovlev design office was worlting out a single-seater fighter Yak-17 and a two-seater fighter Yak-21T(Yak-17UTl). Flying tests of the first training aircraft Yak- 21 started in April, 1947. Although the aircraft flew well, the tests were stopped, because it had already become clear that Yak-21 T(Yak-17UTI) would have better characteristics.