I'm surprised how few people outside of aviation circles have heard of the SR-71 or know more than that it was "a spy plane"--which is like saying the Taj Mahal is "a big building." Unofficially known (but still not well-known) as the Blackbird, the SR-71 flew at record altitude--90,000 feet--and speed--2,193.2 miles per hour. And it could sustain such speed: In 1990, one flew from L.A. to D.C. in 64 minutes. That's 2,144.83 miles per hour, or six-tenths of a mile per second--think ten football fields every second! 
The need for such a swift strategic reconnaissance aircraft became apparent in 1960, when the Blackbird's subsonic predecessor, the U-2, was rendered a sitting duck by Russian radar. The Skunk Works, Lockheed's secretive advanced development division, succeeded in getting a test version--the A-12--off the ground at Groom Lake (a.k.a. Area 51) in April, 1962--development as head-spinningly fast as the aircraft itself.
Even more astonishing was the tech, frequently summed up at the time as "black magic." Topping a long list of revolutionary components were monstrous Pratt & Whitney J58 jet engines and a design that drastically reduced radar signature, pioneering "stealth" technology. If Russian missiles ever did manage to lock on to a Blackbird, it could simply outrun them. Said a sign over the entrance to an SR-71 base, "Though I Fly Through the Valley of Death I Shall Fear No Evil--For I am at 80,000 Feet and Climbing."
The first flight of the the SR-71 took place at Air Force Plant 42 in Palmdale, California, on December 22, 1964. The first SR-71 to enter service was delivered to the 4200th Strategic Reconnaissance Wing at Beale Air Force Base in January, 1966. A total of thirty-two Blackbirds served until 1998, never succumbing to enemy action in 3,551 mission sorties whose contribution to national security can't be overstated.
Palmdale, CA, Dec. 22, 1964
What replaced the SR-71? Satellites, unmanned aerial vehicles (a.k.a. drones), and, perhaps, a new secret aircraft. 50 years before the Blackbird, the Model B--made by the Wright Brothers for the US Army--averaged 44 mph. If aviation technology has continued at its Wrights-to-Blackbird pace, there's something up there now doing 100,000 mph.
photos: Lockheed-Martin
To learn more about the Blackbird, check out this terrific Code One history or the book Sled Driver, or the photo-intensive online Blackbird museum, habu.org. Or, better still, marvel at a Blackbird in person.