After the setback of the 1925 Schneider Trophy at Baltimore, when the Supermarine S.4 monoplane had crashed during a test flight as it rounded the first marker, and the lack of time to design a new machine for the 1926 competition, Britain had to wait until 1927 for its next event entry.
Mitchell set about a comprehensive series of wind tunnel and tank tests at the Royal Aircraft Establishment and National Physical Laboratory. As these results became available he worked toward the final form of the new S.5.
Although it was felt that the Napier engine had been developed as far as possible, and that a Rolls-Royce engine might produce higher speed, Mitchell decided to keep with the Napier Lion VII for one more year. The engine, producng 900hp, was closely cowled and fitted into the streamlined nose of the S.5. The space in the fuselage was so limited that part of the fuel was carried in the starboard float, where it helped to counteract the huge torque of the propeller on take-off. For the same reason the starboard float was offset 8 in. further from the fuselage centre line than the port float. Everything in the S.5 was designed to reduce drag, with a frontal area as small as possible and rivet heads finished flush with the surface.
Sir Hugh Trenchard, Marshal of the RAF, was fully aware that the British defeat in the 1925 Schneider Trophy was mainly due to the technical superiority of the other competitors, but also that the lack of team organisation played a part. Consequently, in May 1927, when the Air Ministry decided that it would finance and organise the 1927 British entry, an RAF High Speed Flight was formed to operate and fly the aircraft in the race.
In early July, Flt Lt Webster flew the S.5 for the first time and after flying at over 275mph, he recorded in his diary, "Very nice. No snags." Once it was airborne the aircraft performed well but it was not easy to get it off of the water. This was partly due to a dipping port float, caused by the propeller torque, forcing the plane to turn left and spray from the bow wave that made it almost impossible for the pilot to see.
The 1927 Schneider Trophy took place at Venice and the 50km triangular course, which had to be lapped seven times, covered the length of the Lido, with a sharp turn at each end. The race was scheduled for Sunday, 25th September but was postponed to the following day after strong winds and a swell made racing impossible. Only two countries had entered; the defending Italians in Macchi M.52s piloted by M de Bernardi, F Guazetti and A Ferrarin, and the British with two S.5s, N219 and N220, piloted by Flt Lts Worsley and Webster respectively, and a Gloster IVB flown by Flt Lt Kinkead.
Kinkead had drawn the right to go first and recorded an opening lap of 266.5mph in his Gloster. However Bernardi, who followed him, reached a speed of 275mph and the British officials, watching from the roof of the Excelsior Hotel, realised that only the S.5s would have a chance of beating the Italian's high first-lap speed.
Webster in N220 was next away, followed by Guazetti, then Worsley in N219. All Italian eyes were fixed on the scarlet Macchis when it began to rain. Almost at once disaster struck the Italians. Ferrarin's plane came down in a cloud of smoke as he dived toward the shore. Soon afterwards, Bernardi was forced to retire with engine trouble, leaving the three British pilots racing the one remaining Italian, Guazetti.
After the fifth lap, Kinkead was forced to retire with engine trouble after a loose strip of metal had wrapped round the propeller blade. This just left the two S.5s flying against the Italian, who soon came to grief when one of his fuel tanks was punctured and he came down on the lagoon, narrowly missing the Excelsior Hotel.
The British pilots carried a simple but effective device for recording each lap as they completed it. It consisted of a small board with holes cut in it which were covered with paper, one of which was punched out after each lap. Flt Lt Webster thought he might have made a mistake with his hole punching and ended up doing an eighth lap. His S.5 had won the race and his average speed of 281.66mph set up a new world speed record for seaplanes and landplanes.
After the Schneider Trophy victory, the RAF pilots became public heroes, but outside of Southampton there was little mention of Mitchell. His total lack of vanity and self-importance led some people to believe that the S.5 was the result of a lucky chance. Only the members of his team knew of the months of concentrated effort and dedicated work that had been needed to design the fastest aeroplane in the world.