In 1947, the United States Army Air Forces (later the United States Air Force) ordered the improved Bell Model 47A. Most were designated YR-13 and three winterized versions were designated YR-13A. The United States Army first ordered Bell 47s in 1948 under the designation H-13. These would later receive the name Sioux.

Dwight D. Eisenhower became the first U.S. president to fly in a helicopter on July 12, 1957 using the H-13J. The Air Force purchased two modified H-13J's for evacuating the President to a remote command post in the event of a military confrontation with the Soviet Union. The H-13J also shuttled President Eisenhower to Washington's National Airport and his personal retreat in Gettysburg, Pennsylvania. Larger, more comfortable models soon replaced it in presidential transport duties.

The H-13 was used as observation helicopter early in the Vietnam War, before being replaced by the OH-6 Cayuse.

The Bell 47 was ordered by the British Army as the Sioux to meet specification H.240, with licensed production by Westland Helicopters. In order to comply with the terms of its licence agreement with Sikorsky Aircraft, which prevented it building a U.S. competitors aircraft, Westland licensed the Model 47 from Agusta, who had purchased a license from Bell. The first contract was for 200 helicopters. The first 50 helicopters of the contract were built by Agusta at Gallarate in Italy followed by 150 built by Westland at Yeovil. The first Westland Sioux made its maiden flight on 9 March 1965.

Originally designed as a commercial helicopter, the HTL Sioux was acquired by the U.S. Navy for training purposes in the late 1940s, and were in use until retirement in April 1964.  HTLs were used in the Apollo program to familiarize astronauts with its auto-rotation sink rate which was similar to the Lunar Excursion Module

Initially, the United States Navy procured several Bell 47s, designated HTL-1, between 1947 and 1958. The United States Coast Guard evaluated this model, and procured two HTL-1s for multi-mission support in the New York Harbor. The most common U.S. Navy version of the 47 was designated the HTL-4, and dispenses with the fabric covering on the tail boom. The U.S. Coast Guard procured three HTL-5s in 1952 (similar to the HTL-4 but powered by a Franklin O-335-5 engine) and used these until 1960.  The Coast Guard procured two of Bell's Model 47G and designated them HUL-1G in 1959.

SOURCE: National Museum of Naval Aviation & Wikipedia