Hueys have been - and still are - in use by every branch of the US military, a number of armed forces in other countries and numerous civilian operators in almost every nation around the globe. Overall, more than 40 variants of the Huey have been built, all based on the Bell models 204, 205, 212, 214 and 412.
Here is a rundown on the most important US military variants (or versions if you like) used in Vietnam:
The Bell UH-1 Iroquois, or the "Huey" as it's better known, began its life as "model 204" on Bell Aircraft Corporation drawing boards when Bell in february 1955 was awarded the contract for a new US Army medical evacuation (medevac) helicopter. Its initial military designation was XH-40, and on October 22nd, 1956 the first of three prototypes took to the air. The XH-40 sported a Lycoming T53 turboshaft engine yielding 700 horsepower and the now classic ski-style landing gear.
In 1957, Bell Aircraft Corporation changed its name to Bell Helicopter Corporation and got out of the fixed-wing business altogether.
The three XH-40 prototypes were followed by six evaluation helicopters dubbed YH-40. With a slightly more powerful engine and a longer cabin than the prototypes, the YH-40 could carry six passengers in addition to the two pilots. The YH-40 evaluation was a success, and the US Army placed an order for 183 helicopters in the summer of 1959. Since this happened before the US Navy/Army/Air Force unified their naming schemes for aircraft in september 1962, otherwise known as the tri-service agreement, the helicopter got the designation HU-1 for "Helicopter, Utility". Out of this arose the now famous nickname "Huey".
The first US Army Hueys came to Vietnam in 1963. This was an improved version designated UH-1B in accordance with the new US tri-service naming convention. The engine was now even more powerful than before (960 hp) and the rotor had an increased wingspan. The cabin could now take seven passengers or 3000 kgs of cargo.
The US Marines bought the Huey in 1962 and designated it UH-1E. This variant had modifications for shipboard stowage, a hoist and different avionics from the Army variants.
After equipping special UH-1Bs with M-60 machine guns and rockets, it became evident that the helicopter couldn't keep up with the transport ("Slick") and medevac ("Dust Off") UH-1A and UH-1B Hueys it was supposed to be escorting. This led to Bell designing the UH-1C, a dedicated Huey gunship with a more powerful engine than the UH-1B and a redesigned rotor. Vietnam units began receiving it in september 1965. The UH-1C saw only limited production before Bell introduced its now legendary AH-1 Cobra gunship based on the UH-1C. Both UH-1C's and AH-1's served in Vietnam throughout the war, utilizing a wide range of rockets, machine guns, grenade launchers and missiles.
In 1969, the UH-1M were introduced, having a 1400 hp Lycoming T53 engine, a mini-gun system as well as equipment for night fighting.
Bell model 205 was introduced into Army service in 1963 as the UH-1D. The UH-1D could hold more fuel and up to 14 passengers or six stretchers because of its longer cabin compared to the UH-1B. The UH-1D - of which Bell delivered over 2000 of to the US Army - would become the primary transport and medevac helicopter for the US forces in Vietnam. An improved UH-1D became the UH-1H in 1967, and kept rolling off the production line until the late eighties. The Bell model 205 is easy to tell apart from the model 204 in that the side doors have two windows instead of one.
US forces lost almost 4900 Hueys in the Vietnam war. Still, the Huey loss rate was less than one in 8000 sorties. This reflects the sheer number of missions the Hueys flew in "the helicopter war".