 Thus modified, the Ki-21 proved superior, and was ordered into production as the "Army Type 97 Heavy Bomber Model 1A", being ordered into production in November 1937.Production aircraft began to enter service in August 1938, supplementing and then replacing the Fiat BR.20 bombers which had been purchased as an interim measure.Several improved versions followed (see below) before the production of the type ended in September 1944.