What was to become the XV-15 program was launched in 1971 at NASA Ames Research Center. After preliminary work, a competition was held to award two research and development contracts for prototype designs. Companies that responded included Sikorsky Aircraft Corporation, Grumman Aircraft, Boeing-Vertol, and Bell Helicopter. R&D contracts were issued to Bell Helicopter and Boeing-Vertol on October 20, 1972. The two companies design proposals were delivered on Jan 22, 1973.

Boeing proposed a design, Model 222 (not to be confused with the later Bell 222 conventional helicopter) in which the engines were in fixed pods at the end of each wing, and a small, rotating pod with the rotor was slightly closer to the fuselage on the wing. This design simplified the engine design by keeping it horizontal at all times, without having very long driveshafts to the tilting rotors.

The Bell design, Bell Model 301, in which the whole wingtip pod rotated between horizontal and vertical, with the engine and rotor assembly fixed together within the pod. This simplified the power transmission but had more complicated requirements for the engine design, and was probably slightly heavier than the Boeing proposal.

After a review of both proposals, NASA selected the Bell 301 for further development, and a contract for further R&D was issued on July 31, 1973.

Extensive engineering and testing took the next four years to complete the development of the aircraft. The first of two Bell XV-15s first flew on May 3, 1977. Both XV-15s were flown actively throughout the 1980s testing aerodynamics and tiltrotor applications for civilian and military aircraft types that might follow, including the V-22 program.

Source: Wikipedia