Under the company designation Lockheed VZ-120, Lockheed designed and developed two prototypes of a VTOL aircraft using turbojet engines to provide direct lift. The centre fuselage formed a giant ejector duct, more than doubling lift. The first of these was flown on 7 July 1962, but it was not until 20 November 1963 that a first successful flight involving transitions from vertical to horizontal flight, and vice versa, were completed. By then redesignated XV-4A, the two prototypes were handed over to the US Army for whom they had been built under contract. In late 1966 Lockheed modified one of the XV-4As to a new XV-4B configuration, the major change being repalcement of the XV-4A's two 1361kg thrust engines by four each of 1368kg thrust. Testing began in August 1968, but when the aircraft was destroyed in an accident in early 1969 further development was abandoned.