The Ki-21 pre-production bomber made its first flight in late 1936. Serial production of its first modification, the Ki-21-Ia, began in 1938, the same year the aircraft made its combat debut. Air units of the Imperial Japanese Army, which were equipped with the Ki-21-Ia, were involved in active combat operations in China. In the spring of 1939, the Ki-21-Ia took part in combat operations against the Red Army on the Halhin Gol River. The aircraft’s combat payload of 1,000 kg was a good indicator of its time and allowed for massive bombing strikes, and the defensive tactics developed by the pilots of these bombers in air battles resulted in their fairly low losses compared to other types of aircraft. After more advanced models were produced, the first modification bombers were transferred to training units, although a certain number of Ki-21-Ia continued to be operated by combat units. American pilots, who gave Japanese aircraft their own names, referred to Ki-21 bombers as ‘Sally’.