In Spring 1934, Gloster embarked on the construction of a single SS.37 prototype.[5] On 12 September 1934, the SS.37 prototype conducted its maiden flight, piloted by Gloster chief test pilot Gerry Sayer.[5] Initially powered by a 530 hp (400 kW) Mercury IV engine, the prototype was quickly reequipped with a more powerful 645 hp (481kW) Mercury VIS engine. During flight tests, the prototype attained a top speed of 242 mph (389 km/h; 210 kn) while carrying the required four .303 in (7.7 mm) machine guns (two synchronised Vickers guns in the fuselage and two Lewis guns under the lower wing).[5] According to aviation author Francis K. Mason, the Air Ministry adopted a sceptical attitude towards the aircraft achieved such performance from a radial engine design, a thus funded a protracted series of evaluation trials.[5]