Born in the spy-vs.-spy cauldron of the Cold War, the iconic SR-71 "Blackbird" remains the fastest air-breathing military aircraft the world has known. It flew so high and so fast that enemy defenses were powerless to intercept it. Eventually, satellite technology and advanced radar eroded its advantage. In 1998, the U.S. Air Force retired it. Now, with regional threats growing and portable surface-to-air missiles evolving, engineers have once again set out to build the fastest military jet on the planet.

This time, it will take the form of a 4,000-mile-per-hour reconnaissance drone with strike capability. Known as the SR-72, the aircraft will evade assault, take spy photos, and attack targets at speeds of up to Mach 6. That's twice as fast as its predecessor.