American Frank Piasecki was an early pioneer of the twin tandem rotor concept on helicopters. The Prototype XH-21 first flew in 1952. Between 1954 and 1959. it had a top speed of 127mph (204km/h) with a cruise speed of 98mph (158km/h). It had a range of 265 miles (427 km). It could carry 20 combat troops or 12 stretches.

Three hundred Piasecki H-21C Shawnee general-purpose helicopters were delivered to the US Army. Many of them were soon to see service in Vietnam prior to being replaced by of more modern helicopters such as the Bell "Huey" UH-1 general purpose helicopter, CH-47 Chinooks, CH-53 Stallions and CH-54 skycranes.

The USAF Piasecki H-21B was the troop carrying version and was capable of carrying 20 troops. The Piasecki H-21A versions was a Search and rescue SAR helicopter. In the US Air Force the H-21 was known as the WorkHorse but in the Army it was called the Shawnee after their naming protocol of using North American First Nation tribal names. To the rest of the world it became known as the "Flying Banana" because of its shape. The upward angle of the aft fuselage that ensured the large rotors did not hit each other in flight.

The Piasecki H-21 Workhorse/Shawnee helicopter was originally developed by as an Arctic rescue helicopter. The H21 had been designed with winterization features permitting it to be operated and maintained in temperatures as low as -65 degrees F.

French Air Force and Army aviation ALAT, Aviation Legere de l'Armee de Terre H-21C helicopters were subsequently armed with fixed, forward-firing rockets and machine guns. They were used during the French Algerian War. With H-34 ground-attack helicopters acting as escorts the H-21 Flying bananas were used in large-scale counterinsurgency operations many in a troop carrying role.

Other air forces that operated the H-21 helicopter was Royal Canadian Air Force, German Air Force and army, Japan Air Self-Defence Force, Japan Ground Self-Defense Force, Japan Self-Defence Forces, Swedish Air Force and Navy.