Just delivered to the USN for testing. 125477 became the first turbine-powered helicopter in 1951. Photo from: National Museum of the US Navy

A Kaman K-225 was re-engined with a Boeing YT50 turboshaft to become the first turbine-engined helicopter to fly on 11 December 1951. Photo is said to show the maiden flight. The helicopter was owned by the USN but got no military designation. Photo from: Nederlands Instituut voor Militaire Historie

The K-225 was an improved version of Charles Kaman's first helicopter, the twin inter-meshing rotor K-125 and first flew in 1949. Two were purchased by the US Navy and one by the US Coast Guard for testing. One was also evaluated by the US Air Force. 125477 became the first turbine-powered helicopter in 1951 when the Lycoming piston engine was replaced by a Boeing YT-50 turboshaft engine. The helicopter was donated to the Smithsonian in 1957.