Known as the worlds fastest and highest flying aircraft, the SR-71 remains a source of fascination and secrecy over fifty years since its development. In the mid 1950s, the United States Air Force and the C.I.A. sought to replace the U-2. The ARCHANGEL project, spearheaded by Lockheed, led to the development of the A-12, an over-flight aircraft configured to fly at extremely high speeds and altitudes over intended targets. In May of 1960, however, after a U-2 was shot down over the Soviet Union, the United States signed an agreement not to fly manned vehicles over the Soviet Union again. As a result, the newly developed A-12 could no longer complete its mission as originally intended.