In the mid-1930s, the English company Alvis, already operating in the automotive and aeronautical fields, began the development of a new air-cooled nine-cylinder radial engine, completing its first prototype in 1936. After various flight tests carried out before the Second World War, due to the logistical priorities imposed by the conflict, the production of the engine began only in 1947. Under the name Leonides, two different construction series were developed: the 500 series engines for aircraft and those of the 520 series and its derivatives for helicopters. The latter have a large fan for cooling the cylinders and a direct transmission system with a centrifugal clutch. Alvis developed various versions of each series, with optimized characteristics and performance for numerous aircraft. In 1958 the 530 series was introduced, characterized by a larger engine displacement and a maximum power output of 640 HP. The Leonides remained in production up to 1966 and was the last high-power piston aero engine to be produced in the United Kingdom. In Italy some engines of the 500 series were used in the prototype of the four-engine Agusta A.Z.8 in 1958 and in the first prototype of the trainer aircraft FIAT G. 49 in 1953.