[2] Bell believed the YH-40 was ideal for troop transport and cargo carrying as well as the medevac role, a view soon adopted by the Army; they found the pre-production aircraft so much better in service than previous piston-powered helicopters that they soon ordered more.

Although they were intended for evaluation only, the Army quickly pressed them into operational service and Hueys with the 57th Medical Detachment arrived in South Vietnam in March 1962.

[4] Fourteen of the original order of UH-1As were designated as TH-1A which were used for crew training and a single aircraft was redesignated XH-1A for grenade launcher testing in 1960.

[3] The first UH-1As to arrive in Vietnam after the 57th Medical Detachment were with a new test unit, the US Army's Utility Tactical Transport Company (UTTCO).

[citation needed] The UH-1C was specifically developed as a gunship version until the "interim" attack helicopter, the Bell AH-1G Huey Cobra was available and to correct the deficiencies of the UH-1B when it was used in the armed role.

[3][4] The "Charlie" model was fitted with the 1,100 shp (820 kW) T53-L-9 or L-11 engine to provide the power needed to lift the weapons systems in use or under development at the time.

The increased power lead Bell's engineers to design a new tailboom for the "C" which incorporated a wider chord fin on a longer boom and larger synchronized elevators.

[3][4] The earlier "short-body" Hueys were a success, especially in the gunship role, but lacked the cabin space to be an effective troop transport.

The US Army wanted a version that could carry a crew of four (two pilots and two door gunners) and also deliver an infantry section of eight to ten soldiers.

Bell's solution was to stretch the UH-1B fuselage by 41 inches (105 cm) and use the extra space to fit two sideways-facing seats on either side of the transmission.

The enlarged cabin could also accommodate six stretchers, double that of the earlier models, making the "Delta" a good MEDEVAC aircraft.

[3][4] The first Army unit deliveries of the "Delta" model were on 9 August 1963 when the 11th Air Assault Division (Test) at Fort Benning Georgia received two.

[3] Upgrading the UH-1D to the Lycoming T53-L-13 engine, plus relocating the pitot tube from the nose to the roof resulted in a new model, the UH-1H, which was to become the most produced variant of the Huey family.

The major changes included the use of all-aluminum construction for corrosion resistance (earlier UH-1s had some magnesium components), special avionics that were compatible with Marine Corps ground frequencies, a rotor brake to stop the rotor quickly on shutdown for shipboard use and a roof-mounted rescue hoist.

This meant the later "Echo" Hueys were completed with the "Charlie" style tailboom including the larger fin, the 540 rotor system and a gross weight of 8,500 lb (3,900 kg).

Many were upgraded to the Lycoming T53-L-13 engine which produced 1,400 shp (1,000 kW), making these "Echos" similar to the Army "Mike" models.

The USAF already had a large inventory of these engines for its fleet of Sikorsky HH-3 Jolly Green Giant rescue helicopters and wanted to have commonality between the types.

The UH-1H was an improved UH-1D, with the Lycoming T53-L-13 engine of 1,400 shp (1,000 kW) installed, plus the pitot tube relocated from the nose to the roof, to reduce ground damage to it.

The aircraft incorporated minor changes for safety, such as dual sliding door locks and a baggage compartment in the tailboom.

[3] The "Hotel" model Huey was also the basis for a number of sub-variants: A UAV version of UH-1H is being developed by Aurora Flight Sciences using the Tactical Autonomous Aerial Logistics System platform for the Autonomous Aerial Cargo Utility System (AACUS) program.

[7] The Bell Huey II is a re-manufactured, modified and re-engined military UH-1H, including an Allison T53-L-703 turboshaft engine of 1,800 shp (1,343 kW), a vibration-reduction system, infrared countermeasures and a night-vision-goggle (NVG) compatible cockpit.

[3] With deliveries of the UH-1E underway to the US Marines the US Navy tested a few and found that the type would suit their uses as a utility helicopter.

[3][4] The “Lima” Huey was basically a UH-1E, of the later production type based on the Army's UH-1C, equipped with a rescue hoist and intake particle screen.

[3][4] The first four “Limas” were delivered in November 1969 and sent to Vietnam to equip the US Navy HA(L)-3 detachment assigned to Operation Sealords.

The Canadian government provided the incentive for Bell to continue development of the Twin Huey using Pratt & Whitney Canada PT6T twin-pack engine.

There was an HH-1N version produced for the USAF as a base rescue helicopter and for use by the 20th Special Operations Squadron in the counter-insurgency role using the call sign Green Hornet.

[4] An unknown number of UH-1Fs were modified to UH-1P configuration by the USAF for use by their 20th Special Operations Squadron, The Green Hornets, based in southeast Asia during the Vietnam War.

The "Papa" Hueys were in fact modified and employed as gunships armed with machine guns and rocket launchers.

The aircraft's skid landing gear was modified to retract allowing the radar antennae to rotate while in flight.

[3] At least four UH-1Hs were modified to JUH-1s for use by the US Army in Europe and Korea for use as technology demonstrators in the Stand Off Target Acquisition System (SOTAS) program.