In 1966 McDonnell company in view of the lack of future General Dynamics/Grumman F-111B fleet defence fighter program put forward an unsolicited proposal for a variable-geometry version of their world-beating Phantom. The idea was to produce a faster and less expensive alternative to a clean sheet design like the one eventually procured by the US. Navy, the F-14A Tomcat. While they were at it, McDonnell people decided to evolve two versions: the original F-4J(FV)S based on the F-4J for the US. Navy, and with the British market in mind, the F-4M(FV)S based on the F-4M for the Royal Air Force. Later even bolder re-engined configurations were studied. To no avail, neither the US. Navy nor the RAF saw potential in the variable-geometry Phantom to compete against brand new designs.
