The Army designated this prototype the XH-40 and the first one flew on October 22, 1956. They joined with Lycoming to develop the XT-53 to Bell Helicopter specifications, and this new engine eventually powered the prototype Huey. Lycoming had not designed this engine for a specific application but Bell engineers saw great potential in this power plant. At about the same time, the Lycoming Company was developing the XT-53 engine with Army backing. Testing the H-13D gave Bell the confidence to incorporate a turbine engine in the Army's new transport helicopter design. Compared to the reciprocating piston engine, the turbine was lighter, smoother, easier to maintain, and much more reliable. The turbine engine represented a revolutionary step in the development of helicopters, and the Army saw it as a critical component in the new medevac helicopter design. Bell called this modified H-13D test aircraft the Bell Model 201 but the Army gave it the designation XH-13F. Bell was already flying an H-13D equipped with a French-designed, American-made, Continental XT-51 gas turbine engine. The Army held the company in high regard, based on the excellent service rendered by the piston engine-powered Bell 47 during the Korean War.

The helicopter could climb to a service ceiling of 1,824 m (6,000 ft).Д«ell Helicopter Corporation enjoyed several advantages in competing for this contract. It could carry a payload of 360 kg (800 lb) a distance of at least 365 km (227 miles) and cruise at 184 kph (114 mph). The Army specifications described an aircraft that weighed 3,600 kg (8,000 lb), fully loaded. Army launched a design competition for a new medical evacuation (medevac) helicopter. The concept for this aircraft sprang from the cold, muddy battlefields of the Korean War, where the original MASH (Mobile Army Surgical Hospital) helicopter, the Bell 47, recovered thousands of wounded soldiers and delivered them straight to critical care units. They usually heard the unmistakable whop-whop-whop of the main rotor blade long before they saw a Huey. People knew it not just on sight but by sound, too. For a time, the Huey was one of the most recognizable aircraft in history. Army as a derivative of the original designation HU-1A - Helicopter, Utility, Model 1A. military operated them and they ranged to every corner of South Vietnam and into Cambodia and Laos. What the jeep was to Americans during World War II, so was the Huey to those who fought in Vietnam.

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