Work began in 1956 on development of a fully tracked armored personnel carrier (APC) to equip the Japan Ground Self-Defense Force, with two prototypes being completed by Komatsu (the SU-I) and by Mitsubishi (the SU-II) in 1957. These two prototypes were evaluated against the American M59. They were followed by a second series of 11 prototypes, including both APCs and carriers for 81-mm and 107-mm mortars, with a third series of four prototypes being completed and tested in 1959.[3]