In May 1937 a specification was issued for a two-seat co-operation aircraft for the Japanese Army and to meet this Mitsubishi put forward the Ki-35 and Tachikawa the Ki-36. In the event only Tachikawa received an order for a prototype to be built, the aircraft being designed by a team

The ST.4 was aimed at the market for a light transport feeder liner and for operation by private owners, being a twin-engine light low-wing monoplane with Pobjoy radial engines.