The H-13/HTL-4 was a two-seat light utility helicopter used by the US Army and Marine Corps during the Korean War.
Originally developed as the Bell Model 47 a prototype first flew on 8 December 1945. The US Air Force adopted the Bell 47 in 1947 with the designation YR-13. The US Navy and Marine Corps also adopted it in 1947 and designated it the HTL-4. The US Army adopted it in 1948 and designated it the H-13 Sioux.
US Marine Corps HTL-4s arrived in Korea as part of Observation Squadron 6, VMO-6, and they served in Korea throughout the war. Four US Army H-13Bs of the 2nd Army Helicopter Detachment arrived in Korea in late December 1950 and were the first US Army helicopters to serve in Korea. They proved their value during their first month of operations by evacuating more than 500 casualties from the combat area during January 1951. They proved so valuable for medical evacuation that in 1952 fifteen H-13Bs were converted to carry external stretchers and were designated H-13Cs. The other main use of H-13Bs during the Korean War was for battlefield observation and surveillance.
The Bell Model 47 and its variants were such simple, reliable, and low-cost helicopters that they remained in continuous production from 1946 until 1973 and were used world-wide.