The Huey story traces back over four decades. A great advance in helicopter propulsion had come in the 1950s with the adaptation of the gas turbine engine to helicopter flight. The piston-drive engines used in Korea and on the Army's UH-34 utility helicopters in the 1950s and early 1960s had produced only one horsepower for each three pounds of engine weight. New gas turbine engines had a much more favorable efficiency ratio. This permitted the construction of small, low-profile aircraft. One of the most important milestones during this period was the decision to develop the XH-40 Bell Utility Helicopter and to power it with a turbine engine. Although designed as an aerial ambulance, it was recognized even then that this machine might turn out to be the most useful aerial platform ever put in production. In 1955, with an interest in a utility helicopter designed around a turboshaft engine, the Army had the Air Force develop a new helicopter for its use. At that time the Army did not have its own aircraft development capability. The design competition was for the first helicopter for the Medical Service Corps, whose primary mission was to be used for aeromedical evacuation. The winner of this competition was Bell Helicopter Company with its model, the XH-40. The design selected, Bell's Model 204, was to be powered by a new Lycoming T-53 engine of some 850 shaft horsepower and featured a typical Bell two-blade teetering rotor. This was the first turbine-powered helicopter for the Army. First flight of the new design was in October 1956, development and production following in 1959. The early Bell XH-40 had been standardized as the HU-1 and was envisioned then as the replacement for the L-20 utility airplane and the H-19 utility helicopter. Further growth versions of the Bell machine were planned to replace the bulk of the missions then performed by the Sikorsky H-84 and the Vertol H-21. In the original helicopter designation series, the first three aircraft receiv...