The pre-production sample of the Ki-21 bomber made its first flight at the end of 1936. Serial production of its first modification, Ki-21-Ia, was started in 1938, the same year the combat debut of the aircraft took place. Aviation units of the Ground Forces of Imperial Japan, armed with the Ki-21-Ia, were involved in active hostilities on the territory of China. In the spring of 1939, Ki-21-Ia participated in combat operations against the Red Army on the Khalkhin-Gol River. The aircraft's combat load of 1,000 kg was a good indicator for its time and allowed for massive bomb strikes, and the defensive tactics developed by the pilots of these bombers in air battles caused their fairly low losses compared to other types of aircraft. After the appearance of more advanced models, bombers of the first modification were transferred to training units, although a certain number of Ki-21-Ia continued to be operated by combat units. Ki-21 bombers were referred to as "Sally" by American pilots who gave their names to Japanese aircraft.
