The Boeing Vertol (model 114) YCH-1B/YCH-47A made its initial hovering flight on September 21, 1961. The all-weather medium lift CH-47A Chinook first entered service in Vietnam about 1966. The CH-47A was powered by either AlliedSignal Engines T55-L-5 (2200 shp) or T55-L-7 (2650 shp) engines. CH-47B was powered by two AlliedSignal Engines T55-L-7C (2850 shp) engines. Models CH-47A, CH-47B, and CH-47C, all utilized the same airframe, but later models featured upgraded engines. The CH-47B featured a blunted rear rotor pylon, redesigned rotor blades, and strakes along the rear ramp and fuselage to improve flying characteristics. The Boeing Vertol (model 234) CH-47C had a strengthened transmission, AlliedSignal Engines T55-L-11C (3750 shp) engines, and increased range. The CH-47C could carry from 33 to 44 troops or 24 litters plus two medical attendants. All three models saw wide use during the Vietnan war. They replaced the H-21 Shawnee in the combat assault role. The Chinook has two three-bladed main rotors, two engines, and a speed of 173 mph (150 knots).
The CH-47B was the standard troop transport used by the 1st Cavalry Division in Vietnam. The Chinook could be equipped with two door mounting M60D 7.62mm machine guns on the M24 armament subsystem and a ramp mounted M60D using the M41 armament subsystem. A few CH-47A Go-Go Birds were also used experimentally in the gun ship role. Some CH-47 "Bombers" were equipped to roll-out riot control gas or napalm onto Viet Cong bunker complexes from the rear cargo ramp. The CH-47 could be equipped with a hoist and cargo hook. The Chinook proved especially valuable in "Pipe Smoke" aircraft recovery missions. The "Hook" recovered about 12,000 aircraft valued at over $3 billion during the war. Boeing-Vertol (models 114/234) were sold in 13 countries and built under license in Italy. The current versions of the Chinook are the CH-47D and CH-47F.
The MH-47 variant of the H-47 are intended for special forces operations and have in-flight refuelling, a fast-rope rappelling system and other upgrades.