The Light Helicopter Experimental (LHX) program was a United States Army helicopter procurement project to replace existing helicopters. The U.S. Army started the LHX program in 1982 to replace the UH-1, AH-1, OH-6, and OH-58 helicopters.
Early concepts included both utility and scout/attack versions, but by 1988 the utility concept was dropped. A request for proposals for the new helicopter type was released in June 1988. In October 1988, the Boeing-Sikorsky and Bell-McDonnell Douglas teams received contracts for their designs.
The Bell-McDonnell Douglas LHX design featured ordnance carriage within a combined wing/sponson and instead of a tail rotor used a NOTAR ducted exhaust system. The Boeing-Sikorsky design carried ordnance within upward swinging gull-wing doors in the sides of the fuselage and featured a fenestron tail rotor.
In April 1991, the Boeing-Sikorsky team was selected as the contest winner and received a contract to build four prototypes of their design, designated the "RAH-66 Comanche" by the Army.
Source: Wikipedia, "Light Helicopter Experimental", available under the CC-BY-SA License.