The Firefly was designed as a shipboard two-seat fighter and reconnaissance aircraft around the then new Rolls Royce Griffon engine by Herbert Chaplin and the prototype (Z1826) flew for the first time on 22 December 1941.

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.