The term penetration fighter was used for a short time to describe a theoretical long-range fighter aircraft designed to penetrate enemy air defences and attack defensive interceptors. The concept is similar to the escort fighter, but differs primarily in that the aircraft would not operate in close concert with bombers. This was, in effect, the same role played by the North American P-51 Mustang during World War II, whose presence above Germany allowed USAAF bombers to fly at will over the country.