The Hughes AIM-4 Falcon was the first operational guided air-to-air missile of the United States Air Force. Development began in 1946; the weapon was first tested in 1949. The missile entered service with the USAF in 1956. Produced in both heat-seeking and radar-guided versions, the missile served during the Vietnam War with USAF McDonnell Douglas F-4 Phantom II units. Designed to shoot down slow bombers with limited maneuverability, it was ineffective against maneuverable fighters over Vietnam. Lacking proximity fusing, the missile would detonate only if a direct hit was scored. Only five kills were recorded. With the AIM-4's poor kill record rendering the F-4 ineffective at air-to-air combat, the fighters were modified to carry the USN-designed AIM-9 Sidewinder missile instead, which was already carried on USN and USMC F-4 Phantom II and F-8 Crusader jet fighters. The Sidewinder was much more effective and improved versions continue to serve the armed forces of the United States and numerous allied nations to this day. Development Defensive concept Development of a guided air-to-air missile began in 1946. Hughes Aircraft was awarded a contract for a subsonic missile under the project designation MX-798, which soon gave way to the supersonic MX-904 in 1947. The original purpose of the weapon was as a self-defense weapon for bomber aircraft, which would carry a magazine of three missiles in the rear fuselage, and fire them through a long tube that led through the area that normally held the tail turret. In the case of the B-52, the missile contained a tuner for the bomber's A-3 rear-facing radar, and would follow the signal being reflected off the target aircraft using a semi-active radar homing (SARH) system. Anti-bomber development At the same time that the original MX-798 had been released, a specification for a forward-firing missile for fighter aircraft had been released as MX-799. This had progressed to the point of testing prototype rounds, as the AAM-A-1 Firebird, when its subsonic speed and manual guidance were realized to be serious problems. The project was cancelled, and the recently released MX-904 was redirected to replace Firebird in the anti-bomber role. At this stage the weapon was still designed to be fired out of a tube, now leading from a weapon bay behind the nose-mounted radar with the launch tube exiting below the radar antenna. Instead of a magazine with multiple missiles, three missiles were placed in the tube tip-to-tail. Housing