The US Army awarded Piasecki a contract for the H-21C Shawnee in 1952. Deliveries began in Aug 1954. A US Army H-21 named Amblin Annie made the first non-stop transcontinental helicopter flight across the United States, a distance of 2610 miles, in 37 hours on 24 Aug 1956 using in-flight refuelling.
US Army H-21s, which could carry 20 troopers each, were sent to Vietnam in Dec 1961. Despite its usefulness as an assault and cargo carrying helicopter, it was vulnerable to ground fire due to a slow speed and poor protection of its vital internal systems. These lessons were learned for the UH-1
Huey in 1963 and later, the CH-47
Chinook which were more robust and had replaced it in-theatre by 1964.
Of the 334 H-21s for the US Army, many of which were at H-21C standard (and known elsewhere as the Vertol Model 43), about 140 were passed on to foreign military services. Most had been withdrawn from US Army service by 1966.
The CH-21B/C Shawnee helicopter was not as vunerable as previously believed. The issue that has led to its being vunerable is due to its Wright Cyclone R-1820-103A radial engine which was never designed to operate at the high rpm and manifold pressures required when carrying maximun allowable loads. Engine failures were commonplace, most commonly brought about by failure of the master rod bearing. The CH-21 was faster than either the UH-1A/B helicopters. The UTT Company which was equipped with both UH-1 models when loaded were unable to keep up with the 21. Usually we (those flying the H-21) were asked to slow down so they could protect us. The standard load in Vietnam was 11 ARVN soldiers plus a crew of four. In the delta the standard fuel load to begin operations was 1300 pounds.