Rolls-Royce Limited increase its own production rates. By the late 1920s, aero engines made up most of Rolls-Royce's business. Henry Royce's last design was the Merlin aero engine, which was first flown in prototype form in 1935, although he had died in 1933. This was developed from the R engine, which had powered a record-breaking Supermarine S.6B seaplane to almost in the 1931 Schneider Trophy. The Merlin was a powerful supercharged V12 engine and was fitted into many World War II aircraft: the British Hawker Hurricane, Supermarine Spitfire, de Havilland Mosquito (twin-engine), Avro Lancaster (four-engine) (a development of the Avro Manchester with
Rolls-Royce Crecy the Ricardo E65 engine achieved the equivalent of for the complete engine. By June 1945 a total of 1,060 hours had been run on the V12 engines with a further 8,600 hours of testing on the V-twins. The fate of the six Crecy engines remains unknown. The Crecy proved a unique exercise and Rolls-Royce did not develop any other two-stroke aero engines, the whole concept of advanced piston engines at that time being overtaken by the advent of the practical jet engine. In the Summer of 1941 Supermarine Spitfire Mk II "P7674" was delivered to Hucknall and fitted with a