The H-21 cargo helicopter with tandem rotors was designed for the US. Army during the 1950s and it was used in substantial numbers during the initial phase of the Vietnam War. The H-21 Shawnee was the fourth of a line of tandem rotor helicopters designed by Piasecki. Although the overall dimensions of the YH—21 and the HRP were almost identical and somewhat similar in appearance, the YH—21 weighed twice as much empty (9,148 pounds), and had three times the horsepower (1,425) and useful load carrying capability (5,556 pounds), while retaining approximately the same air speed. The Piasecki Helicopter Corporation, its new name since 1946, developed the YH—21 as an Arctic rescue helicopter for the Air Force.

The Boeing Vertol (formerly Piasecki) H-21 was a multi-mission helicopter, utilizing wheels, skis, or floats. It was used for Artic rescue because it performed so well at low temperatures.

The CH-21 also served with the U.S. Air Force (as the "Workhorse"), the French Navy, the Royal Canadian Air Force and the West German Air Force. The French used an armed version of the CH-21 in Algeria, mounting guns in the door ways and on the skids.

The CH-21B assault helicopter could carry 22 fully-equipped troops, or 12 stretchers, plus space for two medical attendants, in the MedEvac role. The CH-21B was first deployed to Vietam in December 1961 with the Army's 8th and 57th Transportation Companies, in support of ARVN (Army Vietnam) troops. The CH-21B/CH-21C Shawnee could be armed with 7.62mm or 12.7mm door guns. Some Shawnees were armed with flex guns under the nose. An interesting experimental version was tested stateside with a Boeing B-29 Superfortress ball-turret mounted beneath the nose.

The CH-21 was relatively slow. It's cables and fuel lines were so vulnerable to small arms fire it was even rumored that a CH-21 had been downed by a Viet Cong spear. The Shawnee was the "Workhorse" of Vietnam until 1964 when it was replaced with the fielding of the UH-1 "Huey" in 1963, and the later fielding of the CH-47 Chinook in the mid-1960s. The Shawnee had two tandem fully-articulated three-bladed counter-rotating rotors. The CH-21 was powered by one Curtis-Wright R1820-103 Cyclone supercharged 1150 hp piston engine. The CH-21B was equipped with an uprated 1425 shp engine. The CH-21 had a speed of 128 mph (111 knots)

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