The Sikorsky UH-60 Black Hawk is a medium-lift utility or assault helicopter derived from the twin-turboshaft engine, single rotor Sikorsky S-70. The S-70 was the winner of a United States Army Utility Tactical Transport Aircraft System (UTTAS) competition in the early 1970s to replace the UH-1 Huey family. Though the two final competing designs were each developed to Army specifications, the UH-60 was selected over the Boeing-Vertol YUH-61 entry from Boeing-Vertol. It would go on to serve as the basis for variants in service with other branches of the US military.