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 . The tailboom was also lengthened, in order to accommodate the longer rotor blades. Altogether, the modifications resulted in a gross weight capacity of . 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. During 1966, Bell installed the Lycoming T53-L-13 engine to provide more power for the helicopter. 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. Marine Corps In 1962, the United States Marine Corps held a competition to choose an assault support helicopter to replace the Cessna O-1 fixed-wing aircraft and the Kaman OH-43D helicopter. The winner was the UH-1B, which was already in service with the Army. The helicopter was designated the UH-1E and modified to meet Marine requirements. The major changes included the use of all-aluminum construction for corrosion resistance, radios compatible with Marine Corps ground frequencies, a rotor brake for shipboard use to stop the rotor quickly on shutdown and a roof-mounted rescue hoist. The UH-1E was first flown on 7 October 1963, and deliveries commenced on 21 February 1964; a total of 192 Iroquois of this model were completed. Due to production line realities at Bell, the UH-1E was produced in two different versions, both with the same UH-1E designation. The first 34 built were essentially UH-1B airframes with the Lycoming T53-L-11 engine producing 1,100 shp (820 kW). When Bell switched production to the UH-1C, the UH-1E production benefited from the same changes. The Marine Corps later upgraded UH-1E engines to the Lycoming T53-L-13, which produced 1,400 shp (1,000 kW), after the Army introduced the UH-1M and upgraded their UH-1C helicopters to the same engine. Air Force The United States Air Force (USAF) held a competition for a helicopter to be