The Air Ministry issued Shorts with contract number 672299/37, under which a pair of prototype S.29s were ordered. However, prior to this, Shorts had decided to undertake a successful practice which had been performed with the earlier Empire flying boat in producing a half scale version of the aircraft, known as the "S.31" (also known internally as the M4 – as per the title on the tailfin), to prove the aerodynamic characteristics of the design. The S.31, which was largely composed of wood, was powered by an arrangement of four Pobjoy Niagara engines and featured a retractable undercarriage, operable bomb bay doors, and other measures to realistically represent the larger production aircraft. It was constructed at Short's Rochester facility.