In 1940, after receiving a request in March of that year from the Ministry of Aircraft Production for a high-rated ( 40000 feet ) Merlin for use as an alternative engine to the turbocharged Hercules VIII used in the prototype high-altitude Vickers Wellington V bomber, Rolls-Royce started experiments on the design of a two-stage supercharger and an engine fitted with this was bench-tested in April 1941, eventually becoming the Merlin 60.