The first operational air-to-air missile (AAM) was the AIM-9 Sidewinder, which started development in 1950 at China Lake, California. The project was designed by China Lake personnel including Dr. Bill McLean and Dr. Walter LaBerge. The first drone was shot down in 1953 by test pilot Wally Schirra at the China Lake test ranges. Schirra went onto to become a NASA Mercury, Gemini and Apollo astronaut. The AIM-9B first made a combat kill in 1956 in Korea. The basic Sidewinder has kept up with constant improvement; the AIM-9X entered production in 2004 as representative of the fifth generation of its family of AAMs.
"Sidewinder" is the name of a poisonous snake that uses heat sensing to find its prey, and its guidance mechanism was the inspiration for the AIM-9. At the time of its development by the U.S. Navy, the U.S. Air Force assumed that radar-guided missiles were superior, but the Air Force entry, the AIM-4 Falcon, soon dropped from consideration.
The AIM-9 has had an exceptionally long history, with many versions, some in evolutionary dead ends, some that were only in limited production, and some that were deliberately restricted performance for export to less trusted countries.
AIM-9 missiles have always been intended for close-range air combat, as opposed to the beyond visual range (BVR) capability of radar-guided missiles. The AIM-9B could only be fired at in a narrow arc behind its target, so the missile would home on the hot jet exhaust; C and D models were incremental Navy improvements. AIM-9E was the first build that included Air Force requirements, and became a standard to which various European B-models were converted. Still, there were separate Air Force and Navy versions for some time.
Finally, the L model met both services' requirements. It was the first "all aspect" Sidewinder, which can be fired from any direction, sensing the heat difference between the entire target and the air surrounding it. L versions went into production in 1978, and were a key British advantage in the Falklands War. That production line converted to the M version in 1982; it was the Operation DESERT STORM combat Sidewinder.
While there was a search for a new short-range version, the mainline versions jumped from R, an incremental improvement on the M, to the current AIM-9X. T through V suffixes were skipped.
The latest version, AIM-9X, is into its Block II revision. It can take cueing from a Joint Helmet-Mounted Cueing System (JHMCS) sight mounted on the pilot's helmet; the pilot literally only has to look at the target, and the missile will know where it is to go.
In many respects, the AIM-9X is an improved AIM-9M. They have the same rocket motor and warhead, but a new airframe, infrared seeker, and steering mechanism. F-22 Raptor and F-35 Joint Strike Fighter carry their weapons internally for stealth, and, while the X can be fired from existing launch rails, the more compact airframe fits better into internal bays. Its aerodynamic control has taken a major jump in performance, and the X is much more agile than any previous Sidewinder.
AIM-9X infrared seekers are being used as the terminal guidance seeker for the NCADE missile, initially intended as an air-launched anti-ballistic missile but with a wide range of possible other applications.