The British glider development started in mid-1940, prompted by the assault on Eben Emael. Among the types developed were the 28 trooper Airspeed Horsa and the 7-ton capacity General Aircraft Hamilcar cargo glider. The Hamilcar could carry vehicles, anti-tank guns and light tanks into action. The General Aircraft Hotspur – originally planned as a compact assault glider carrying a small number of troops – was used for training the British Army pilots who formed the Glider Pilot Regiment. The Slingsby Hengist was a backup design which was not required when the similar capacity American-built Waco CG-4 (given the British service name "Hadrian") became available in large numbers through lend-lease. Four hundred of the 3,600 Horsas built were supplied to the USAAF.