Based on the successful UH-1B Huey model, the UH-1D was an improved and more powerful version that included increased cabin space for up to 14 passengers. As the YUH-1D (taken from the base Bell 205 airframe), the system first appeared in 1960 and was placed into production by 1963.
With over twice the seating capacity of the B and C Hueys, the D model was fitted with an appropriately-powered T53-L-11 turboshaft engine driving the two-blade main rotor and tail rotor sections. The further design - the UH-1H - provided an even more powerful T53-L-13 turboshaft engine, which the US Army saw delivered in 1967.
The D and H models were the two production variants which saw extensive use throughout the Vietnam conflict as MedEvac, close-support, transport or assault. The Vietnam gunship variants could be fitted with a myriad of armaments including high-explosive rockets, machine guns, miniguns and grenade launchers. Door-mounted pintle positions and wing stubs were provided to compliment this impressive armament array. In short, the Huey airframe proved to be quite valuable to the air-only accessible areas of the jungle - necessitating the need for helicopter development to continue in the any modern military.