The SR-71 Blackbird is the arguably the most popular and easily recognizable airframe ever used by the U.S. Air Force. It maintains the speed record it set back in 1976 (even with a broken engine). The Blackbird's missile evasion technique is legendary; it simply flew faster than the whatever was chasing it. Not one SR-71 was ever shot down.
It could take a full photo of the entire country of North Korea in seven minutes and fly across the entire United States, lengthwise, in just over an hour. Not bad, but that capability didn't happen overnight. The Air Force actually developed more than one supersonic plane for its reconnaissance and strike missions.
Here are the four aircraft that were the ancestors of the powerful SR-71 Blackbird
1. XB-70 Valkyrie
Only 2 of North American Aviation's B-70 bombers were ever built, and the program only lasted for the five years between 1964 and 1969. The Valkyrie was a six-engine bomber, capable of flying Mach 3, designed to outrun enemy interceptor aircraft with speed and altitude. At the time, interception was the only defense against bombers.
Surface-to-air missiles changed the game.
The XB-70 was still fast enough to fool radar, but its limited range and expense made the B-52 a more economically efficient choice for production. Though short-lived, the Valkyrie did blaze a trail for the structural dynamics that would be so crucial to the SR-71.
The last XB-70 is on display at the National Museum of the United States Air Force in Dayton, Ohio.
2. Lockheed A-12 "Archangel" or "Oxcart"
Not to be confused with the later naval stealth fighter proposal dubbed the A-12 Avenger II, the A-12 Archangel was a recon aircraft developed by Lockheed for the CIA between 1962 and 1967. The defense giant's "Skunk Works," the nickname given to its Advanced Development Programs department, developed the A-12 for the CIA's Oxcart Operation.
Oxcart was the agency's effort to replace the U-2 spy plane after it became increasingly susceptible to Soviet SAMs. They were wildly successful – the planes boasted a host of new technologies designed just for the program. They were built with titanium to handle hypersonic speeds (strangely obtained from the Soviet Union).
Though designed to fly over Cuba and the USSR, the Lockheed A-12 never executed that mission. It flew over North Vietnam and North Korea during the Pueblo Crisis.
The North Vietnamese were able to track the A-12 via radar, and routinely launched missiles at it. It never took a direct hit from a SAM but did get debris from an exploding missile lodged in its fuselage.
Since the A-12 was never going to fly over the Soviet Union and the use of satellite photography was on the rise, the program was scrapped almost as soon as it had begun. The A-12s were either stored in Palmdale, California, or sent to museums.
The A-12 could fly higher and faster than the SR-71, but the Blackbird's side-looking radar and cameras could see enemy territory without penetrating their airspace.
3. M-21 Drone Carrier
The M-21 variant of the A-12 was designed to carry the Lockheed D-12 Drone. This variation had a cockpit for the drone's launch control officer who released the autonomous drone which was mounted on the back of the M-21 airframe.
The D-21 was launched from the back of the A-12. Once its mission was complete, the drone would eject the data it collected at a preprogrammed point and then self-destruct. The ejected data was caught in mid-air by a C-130.
This program was canceled in 1966 when a drone collided in midair with its launcher. The M-21 crew all bailed out, except for the LCO. From then on, the D-21 would be launched from under the wing of a B-52.
4. Lockheed YF-12
The YF-12 was a twin-seat version of the A-12. Designed to be an interceptor, the YF-12 set the speed records that would only be surpassed by the legendary SR-71. It also has the distinction of being a publicly announced aircraft, which had benefits of keeping the A-12 a secret because the public couldn't tell the difference.
The cost of the Vietnam War kept the YF-12 from the Air Force inventory. And by the time the funds were available, the YF-12 wasn't necessary to defend the mainland U.S., so the program was scrapped.
The aircraft did successfully test the AIM-47 Falcon missile, which was the predecessor to the Phoenix missiles. The YF-12 also tested how AWACS could command bombers in a tactical environment, which later helped the development of the B-1 Bomber.
The YF-12 also tested how engine inlet performance affected airframe for NASA, as well as issues related to propulsion interaction, boundary layer noise, heat transfer under high-mach conditions, and altitude hold at supersonic speeds – all necessary to develop the SR-71, not to mention the Space Shuttle program.