While earlier "short-body" Hueys were a success, the Army wanted a version that could carry more troops. Bell's solution was to stretch the HU-1B fuselage by 41 in (104 cm) and use the extra space to fit four seats next to the transmission, facing out. Seating capacity increased to 15, including crew. The enlarged cabin could also accommodate six stretchers and a medic, two more than the earlier models. In place of the earlier model's sliding side doors with a single window, larger doors were fitted which had two windows, plus a small hinged panel with an optional window, providing access to the cabin. The doors and hinged panels were quickly removable, allowing the Huey to be flown in a "doors off" configuration.

The Model 205 prototype flew on 16 August 1961. Seven pre-production/prototype aircraft had been delivered for testing at Edwards AFB starting in March 1961. The 205 was initially equipped with a 44-foot (13.4 m) main rotor and a Lycoming T53-L-9 engine with 1,100 shp (820 kW). The rotor was lengthened to 48 feet (14.6 m) with a chord of 21 in (53 cm). The tailboom was also lengthened, in order to accommodate the longer rotor blades. Altogether, the modifications resulted in a gross weight capacity of 9,500 lb (4,309 kg). The Army ordered production of the 205 in 1963, produced with a T53-L-11 engine for its multi-fuel capability. The prototypes were designated as YUH-1D and the production aircraft was designated as the UH-1D.

In 1966, Bell installed the 1,400 shp (1,000 kW) Lycoming T53-L-13 engine to provide more power for the aircraft. The pitot tube was relocated from the nose to the roof of the cockpit, to prevent damage during landing. Production models in this configuration were designated as the UH-1H.

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