Rolls-Royce commenced design of a heavy machine gun in early 1940, intended for use in aircraft. The result was a recoil operated weapon firing .50 Browning cartridges. This was evaluated in March 1941 and proved prone to stoppages. A revised gas-operated weapon was designed and built to try and solve these problems.
As well as the .50 inch versions, it was planned to modify the gas operated gun to fire the more powerful .55 inch ammunition used in the Boys anti-tank rifle. All work on the Rolls-Royce machine guns was abandoned in 1942.