In the mid-1950s, the Army sought a new utility helicopter able to carry an 800 lb. payload with a mission radius of 100 nautical miles at a constant speed of 100 knots, and Bell Helicopter's design met those requirements and was selected in January 1955. The prototype XH-40 used a Lycoming XT-53-L-1 turbine engine rated at 700 shaft horsepower, reflecting the Army's view that turbine engines were crucial because they were lighter, more powerful, had simpler drive systems, and did not take up valuable cabin or fuselage space. The helicopter also featured a low-profile fuselage, extensive window space, access panels, aluminum skids, and a transmission in an open bay that GIs called the "hell hole." Following the Army's practice of naming helicopters after Indian tribes, the aircraft was officially named the Iroquois, but soldiers quickly nicknamed it the "Huey" from its initial designation, HU-1. The first production UH-1D troop transport was accepted in May 1963, with increased fuel capacity to 242 gallons for its Lycoming T-53-L-11 turbine engine and large sliding doors with double windows on each side of the cabin. It could carry 24 troops, or a medical attendant and six stretchers, or 4,000 lbs. of cargo, and Bell Helicopters produced 2,008 UH-1Ds between 1962 and 1966. Manufactured in 1965 by the Bell Helicopter Company of Fort Worth, Texas, this helicopter served in Vietnam from February 1966 to May 1967 on medivac duty with the 498th Medical Company, then as a gunship with the 114th Assault Helicopter Company until August 1968. It was later assigned to Lockheed Corporation, served in Germany, transferred in May 1973 to NASA's Langley Research Center in Hampton, Virginia, and while there was modified to UH-1H configuration. It later served at various duty stations and ended up at Fort Rucker, Alabama, from 1978 to 1990, when it was surplused. The 114th Assault Helicopter Company, nicknamed the "Knights of the Air," served in Vietnam for ten years. Its specifications included a gross weight of 9,500 lbs., a maximum speed of 138 mph, a range of 293 miles with combat load, and armament of two XM-23 M60D machine guns.