Until Lockheed Martin finally builds the SR-72 Mach-6 spy plane it announced in November, the iconic SR-71 Blackbird, capable of flying three times the speed of sound, remains the fastest warplane ever flown operationally. So fast that no missiles fired at it had a chance of hitting. When the U-2 spy plane was built in the 1950s, its designer Clarence “Kelly” Johnson already knew that it would be vulnerable to enemy defenses.