The principal design differences between the S.6 and the S.6B were made in its more powerful Rolls-Royce R engine and redesigned floats, providing much needed additional cooling; minor aerodynamic refinements typically aimed at drag reduction were also implemented. A pair of S.6Bs, serials "S1595" and "S1596", were constructed for the competition. Flown by members of RAF High Speed Flight, the type competed successfully, winning the Schneider Trophy for Britain. Shortly after the race, S.6B "S1596", flown by Flt Lt. George Stainforth, broke the world air speed record, attaining a peak speed of 407.5 mph (655.67 km/h).