The HU-1A (later redesignated UH-1A) first entered service with the 101st Airborne Division at Fort Campbell, Kentucky, the 82nd Airborne Division, and the 57th Medical Detachment.
Although intended for evaluation only, the Army quickly pressed the new helicopter into operational service, and Hueys with the 57th Medical Detachment arrived in Vietnam in March 1962.
The UH-1 has long been a symbol of US involvement in Southeast Asia in general and Vietnam in particular, and as a result of that conflict, has become one of the world's most recognized helicopters.
In Vietnam primary missions included general support, air assault, cargo transport, aeromedical evacuation, search and rescue, electronic warfare, and later, ground attack.
During the conflict, the craft was upgraded, notably to a larger version based on the Model 205.
This version was initially designated the UH-1D and flew operationally from 1963.

During service in the Vietnam War, the UH-1 was used for various purposes and various terms for each task abounded.
 UH-1s tasked with ground attack or armed escort were outfitted with rocket launchers, grenade launchers, and machine guns.
As early as 1962, UH-1s were modified locally by the companies themselves, who fabricated their own mounting systems.
These gunship UH-1s were commonly referred to as "Frogs" or "Hogs" if they carried rockets, and "Cobras" or simply "Guns" if they had guns.
UH-1s tasked and configured for troop transport were often called "Slicks" due to an absence of weapons pods.
Slicks did have door gunners, but were generally employed in the troop transport and medevac roles.

UH-1s also flew "hunter-killer" teams with observation helicopters, namely the Bell OH-58A Kiowa and the Hughes OH-6 Cayuse (Loach).
Towards the end of the conflict, the UH-1 was tested with TOW missiles, and two UH-1B helicopters equipped with the XM26 Armament Subsystem were deployed to help counter the 1972 Easter Invasion.
USAF Lieutenant James P. Fleming piloted a UH-1F on a 26 November 1968 mission that earned him the Medal of Honor.

During the course of the war, the UH-1 went through several upgrades.
The UH-1A, B, and C models (short fuselage, Bell 204) and the UH-1D and H models (stretched-fuselage, Bell 205) each had improved performance and load-carrying capabilities.
The UH-1B and C performed the gunship, and some of the transport, duties in the early years of the Vietnam War.
UH-1B/C gunships were replaced by the new AH-1 Cobra attack helicopter from 1967 to late 1968.
 The increasing intensity and sophistication of NVA anti-aircraft defenses made continued use of UH-1 gunships impractical, and after Vietnam the Cobra was adopted as the Army's main attack helicopter.
 Devotees of the UH-1 in the gunship role cite its ability to act as an impromptu Dustoff if the need arose, as well as the superior observational capabilities of the larger Huey cockpit, which allowed return fire from door gunners to the rear and sides of the aircraft.
In air cavalry troops (i.e., companies) UH-1s were combined with infantry scouts, OH-6 and OH-58 aero-scout helicopters, and AH-1 attack helicopters to form several color-coded teams (viz.,
blue, white, red, purple, and pink) to perform various reconnaissance, security, and "economy of force" missions in fulfilling the traditional cavalry battlefield role.

The Army tested a great variety of experimental weapons on the UH-1; nearly anything that could be carried.
The Army desired weapons with large calibers and high rates of fire, which led to the testing of a 20 mm cannon on a large mount bolted to the cabin floor.
The size of the weapon allowed very little room for movement.
The Army further tested a full-size Vulcan cannon firing out the door of a UH-1.
It was capable of firing 2400 rounds per minute, or about 40 rounds per second.
Despite this being a significant reduction from the nearly 100 rounds per second fired by a standard Vulcan cannon, the installation proved "too kinetic" for the UH-1.
Podded versions of the M24 20 mm cannon were tested in combat over Vietnam.
There was a
The HU-1A (later redesignated UH-1A) first entered service with the 101st Airborne Division at Fort Campbell, Kentucky, the 82nd Airborne Division, and the 57th Medical Detachment. Although intended for evaluation only, the Army quickly pressed the new helicopter into operational service, and Hueys with the 57th Medical Detachment arrived in Vietnam in March 1962. The UH-1 has long been a symbol of US involvement in Southeast Asia in general and Vietnam in particular, and as a result of that conflict, has become one of the world's most recognized helicopters. In Vietnam primary missions included general support, air assault, cargo transport, aeromedical evacuation, search and rescue, electronic warfare, and later, ground attack. During the conflict, the craft was upgraded, notably to a larger version based on the Model 205. This version was initially designated the UH-1D and flew operationally from 1963.
During service in the Vietnam War, the UH-1 was used for various purposes and various terms for each task abounded.  UH-1s tasked with ground attack or armed escort were outfitted with rocket launchers, grenade launchers, and machine guns. As early as 1962, UH-1s were modified locally by the companies themselves, who fabricated their own mounting systems. These gunship UH-1s were commonly referred to as "Frogs" or "Hogs" if they carried rockets, and "Cobras" or simply "Guns" if they had guns. UH-1s tasked and configured for troop transport were often called "Slicks" due to an absence of weapons pods. Slicks did have door gunners, but were generally employed in the troop transport and medevac roles.
UH-1s also flew "hunter-killer" teams with observation helicopters, namely the Bell OH-58A Kiowa and the Hughes OH-6 Cayuse (Loach). Towards the end of the conflict, the UH-1 was tested with TOW missiles, and two UH-1B helicopters equipped with the XM26 Armament Subsystem were deployed to help counter the 1972 Easter Invasion. USAF Lieutenant James P. Fleming piloted a UH-1F on a 26 November 1968 mission that earned him the Medal of Honor.
During the course of the war, the UH-1 went through several upgrades. The UH-1A, B, and C models (short fuselage, Bell 204) and the UH-1D and H models (stretched-fuselage, Bell 205) each had improved performance and load-carrying capabilities. The UH-1B and C performed the gunship, and some of the transport, duties in the early years of the Vietnam War. UH-1B/C gunships were replaced by the new AH-1 Cobra attack helicopter from 1967 to late 1968.  The increasing intensity and sophistication of NVA anti-aircraft defenses made continued use of UH-1 gunships impractical, and after Vietnam the Cobra was adopted as the Army's main attack helicopter.  Devotees of the UH-1 in the gunship role cite its ability to act as an impromptu Dustoff if the need arose, as well as the superior observational capabilities of the larger Huey cockpit, which allowed return fire from door gunners to the rear and sides of the aircraft. In air cavalry troops (i.e., companies) UH-1s were combined with infantry scouts, OH-6 and OH-58 aero-scout helicopters, and AH-1 attack helicopters to form several color-coded teams (viz., blue, white, red, purple, and pink) to perform various reconnaissance, security, and "economy of force" missions in fulfilling the traditional cavalry battlefield role.
The Army tested a great variety of experimental weapons on the UH-1; nearly anything that could be carried. The Army desired weapons with large calibers and high rates of fire, which led to the testing of a 20 mm cannon on a large mount bolted to the cabin floor. The size of the weapon allowed very little room for movement. The Army further tested a full-size Vulcan cannon firing out the door of a UH-1. It was capable of firing 2400 rounds per minute, or about 40 rounds per second. Despite this being a significant reduction from the nearly 100 rounds per second fired by a standard Vulcan cannon, the installation proved "too kinetic" for the UH-1. Podded versions of the M24 20 mm cannon were tested in combat over Vietnam. There was a 
Bell UH-1 Iroquois