Designed by R. J. Mitchell as a tender to Air Ministry R.24/31 Specification for a coastal reconnaissance flying boat for the RAF, it was initially turned down but Supermarine proceeded with the type as a private venture first known as the Southampton V. A contract was placed in 1933 for a prototype powered by two 820 horsepower Bristol Pegasus IIIM engines and the type became known as the Stranraer.