However it was necessary to modernize the fleet in view of the creation of the "air cavalry", which then became the icon of the Vietnam war.Specifically the US Army needed a light single-rotor, four place aircraft capable of carrying in addition to the pilot, a fuel reserve for three hours and a payload of 180 kg (400 pounds) at a cruise speed of at least 204 km/h (110 knots).The US Army in fact estimated a global need of about 3'600-4'000 LOH helicopters.
Extreme cold weather test were conducted at Eglin Air Force base at temperatures down to -50°C.As predictable, the confrontation between the finalists highlighted virtues and vices of each model.As established by the US Army contract the helicopter obtained the civil certification (type certificate H3WE) issued by the Federal Aviation Agency on the 30th of June 1964.The new helicopter received FAA designation Hughes Model 369, while later it was marketed as the Model 500.The rotor hub, mask and swashplate drag for example were reduced by a new fairing.
The original flush engine air intake was replaced with a new one placed in front of the mast fairing.
Among these there was the need for a new light helicopter.
During the '50s the US Army successfully tested the rotary-wing potentials and discovered its great versatility.
Furthermore it had to be manufactured at low cost, be easily maintainable, reliable, with specified performance, minimum time between overhaul of at least 1'200 hours and certified by the Federal Aviation Agency (FAA).
The request for proposals also imposed that the engine to be used had to be the new turboshaft Allison T-63 (commonly known under the civil designation Allison series 250), which at that time was produced by the Allison Division of General Motors Corporation.
Its extreme lightness was obtained thanks to its compact dimensions and a series of innovative technical solutions as for example a simplified transmission, the elimination of the heat exchanger used to cool the transmission, the one-piece tail rotor drive shaft with no coupling or bearings and last but not least a new rotor type called Flexrotor.