In the 1930s, the Royal Thai Air Force – RTAF – relied primarily on aircraft manufactured in the United States and Great Britain. The situation changed with the beginning of the Second World War. After the defeat of France in Europe, Thailand decided to seize the moment and regain its territories in French Indochina. There was made an order for the delivery of new American-made aircraft. However, the US decided that strengthening the Thai Air Force could change the situation in the region in an undesirable direction for them, and refused to supply the aircraft. Thailand referred to Japan, which soon delivered several dozen bombers for the RTAF. Among them were nine Ki-21-Ia aircrafts produced by the Nakajima company, in the spring of 1941, they were included in the composition of Kong Bin Koi 6 (6th Aviation Wing). These aircraft did not have time to take part in combat operations against France but were used in Burma against units of the Chinese National Army in the spring of 1942. After the Chinese forces retreated to their territory, the Thai Ki-21-Ia were deployed for a while to perform patrol and reconnaissance missions. Later they served as training and transport aircraft.