The D-21 was a top-secret stealth drone developed by the CIA during the Cold War. Designed to fly at high altitudes and speeds, it was intended to gather intelligence over enemy territory without detection. The D-21 was part of the Blackbird program, which also included the A-12 and SR-71 spy planes. It was one of the first drones designed for stealthy operations, featuring a composite material construction and a special radar-absorbent coating.