The F-35 Lightning II was developed as a platform to replace multiple US Air Force, Marine Corps, and Navy service aircraft (as well in partnership to replace foreign air force aircraft). To this end, it has been built in three distinct variants: the F-35A, F-35B, and F-35C. The F-35 has become the most produced and popular fifth-generation fighter jet and has been exported to US allies around the world. While the F-35 is replacing many 4th and 4.5th generation aircraft, it will not replace them all as the US Air Force is still purchasing 4.5th gen F-15EX Eagle II fighter jets.