In 1967, the Soviet Union revealed the Mikoyan-Gurevich MiG-25 at the Domodedovo airfield near Moscow. The MiG-25 was designed as a high-speed, high-altitude interceptor aircraft, and made many performance tradeoffs to excel in this role. Among these was the requirement for very high speed, over Mach 2.8, which demanded the use of stainless steel instead of aluminum for many parts of the aircraft. The added weight demanded a much larger wing to allow the aircraft to operate at the required high altitudes. However, to observers, it appeared outwardly similar to the very large F-X studies, an aircraft with high speed and a large wing offering high maneuverability, leading to serious concerns throughout the Department of Defense and the various arms that the US was being outclassed. The MiG-23 was likewise a subject of concern, and it was generally believed to be a better aircraft than the F-4. The F-X would outclass the MiG-23, but now the MiG-25 appeared to be superior in speed, ceiling, and endurance to all existing US fighters, even the F-X. Thus, an effort to improve the F-X followed.