For a brief shining moment, the US Air Force was about to fly the fastest, perhaps the meanest fighter jet of all times. The aircraft, the YF-12A, was one of several proposed variants of the SR-71 Blackbird family of high-speed aircraft during the 1960s. However, an operational version of the aircraft never entered production, as world events and the U.S. in change nuclear politics made it a questionable choice. Today, the YF-12A remains the closest the United States has ever come to producing an armed aircraft capable of Mach 3.

On May 1, 1960, the Soviet Union shot down a U-2 spy plane over the USSR with S-75 “Dvina” surface-to-air. rocket . Central Intelligence Agency pilot Francis Gary Powers piloted the high-flying but slow-flying U-2 taking photographs of the Soviet Union. The CIA had hoped that the photographs would help formulate estimates of Soviet military power, but the capture of the powers and the loss of its aircraft meant that the U-2 was no longer invulnerable to Soviet anti-aircraft defenses. A new aircraft was required to continue the overflight program.

One was already in the works. In 1959, Lockheed’s Advanced Development Programs Division, now nicknamed “Skunk Works,” submitted plans to the CIA for a Mach 3 reconnaissance aircraft, codenamed Oxcart. Later named the A-12, it was designed to perform the same mission as the U-2, with belly-mounted cameras capable of photographing vast areas of the ground below. But the A-12 would fly faster and be much harder to detect than its predecessor.

THE A-12 HAD A PIONEER AIRCRAFT DESIGN. The plane was 101 feet long, much longer than most tactical aircraft at the time, and used two new J58 turbojet engines built into the wings, an unusual design – the engines are usually in the fuselage. The two engines, producing a total thrust of 65,000 pounds in afterburner mode, would allow the aircraft to cruise at Mach 3.1 – giving air defense systems like the S-75 very little time to react. This was unprecedented at a time when the fastest fighter the jet at the time, the F-106 Delta Dart, could only sustain Mach 2.6. The large, blended design for its wings and body gave it plenty of internal volume for fuel, resulting in a range of 4,900 miles before needing refueling. The aircraft would fly as high as 95,000 feet – about three and a half times higher than Mount Everest – allowing cameras to capture a vast panoramic view of the landscape below.

The A-12 was the first aircraft designed with stealth in mind. President Dwight D. Eisenhower, who took a keen interest in the program, wanted the plane to be invisible to radar to prevent any embarrassing incidents where a plane was lost while penetrating enemy territory. A CIA study concluded that reducing an aircraft’s radar signature was possible, so Lockheed set to work modifying the A-12’s design, merging the wings and fuselage as much as possible to reduce the number of vertical surfaces. The design also changed its single large, vertical wing, known as the vertical stabilizer, to two smaller inclined stabilizers. The angled stabilizers presented less surface area to incoming radar waves, reflecting less energy back to enemy radars and making the aircraft more difficult to detect.

As the A-12 entered production, Lockheed quietly approached the Air Force about an armed variant of the new jet; Up until this point, the Air Force had been locked out of the effort to build an unarmed spy plane. The Air Force enthusiastically agreed, and three A-12s in production were diverted to a mysterious new program known only as KEDLOCK. The plane would have an important mission in the nuclear age – the elimination of enemy bombers.

THE THREAT OF ATOMIC ATTACK HAS BECOME A REALITY once long-range B-29 bombers dropped an atomic bomb on the Japanese cities of Hiroshima on August 6, 1945, and Nagasaki three days later. When the soviet union became the second country to develop nuclear weapons in 1949, the nuclear threat extended against the US. The Soviet R-7, the world’s first intercontinental ballistic missile, was not yet a real threat, with a range of only 5,000 miles. By the early 1960s, the heavy bomber was still the primary means of delivering an atomic bomb to its target, and the US continued to develop a defensive fighter that could intercept and shoot down any bomber.

In 1953, the Air Force began development of a new interceptor to protect the United States and Canada from Soviet bomber attacks. The North American Air Force XF-108 Rapier was a large fighter aircraft designed to fly at Mach 3 and an altitude of 60,000 feet. From its commanding position high in the sky, the Rapier would encounter Soviet bombers as they attempted to penetrate American airspace via the North Pole. A new highly advanced radar, the AN/ASG-18, would allow the XF-108 to detect bombers 160 miles away and fly up to 500 feet. The Rapier was to be armed with three GAR-9A air-to-air missiles, each with a maximum speed of Mach 4. Each missile would pack a 0.25-kiloton nuclear warhead, ensuring the bomber’s destruction. However, the ambitious program became too expensive, and America’s changing nuclear strategy made the plane less necessary. In 1959, the Pentagon canceled the XF-108 Rapier.

Despite this setback, the prospect of a Mach 3 fighter whetted the Air Force’s appetite for top speed. And despite the loss of the Rapier aircraft, its radar and key weapon system, the AN/ASG-18 and GAR-9 missiles showed promise. The stage was set for a new aircraft to carry the torch. In the deserts of Nevada, Lockheed had the answer to the continuing need for a new high-speed interceptor.

The Lockheed A-12 is a retired, high-altitude reconnaissance aircraft that could fly at Mach 3 and beyond. Lockheed’s Skunk Works built it for the CIA and was the forerunner of the US Air Force’s YF-12 prototype interceptor.

THE KEDLOCK PROGRAM WAS A COVERT EFFORT TO BUILD AN ARMED A-12. Lockheed workers kept three aircraft on the A-12 production floor, but separate from other projects, to secretly modify them. To confuse Soviet spies, they bore the designation A-11, that of an earlier, unsuccessful spy plane.

The YF-12s that the US kept under wraps were very similar to the A-12 externally, except for a rounded conical fairing like that of the F-14 Tomcat fighter jet. This fairing hid the 40-inch AN/ASG-18 radar, which would not fit in the current A-12 nose design. This ended up giving the YF-12 a dramatically different fighter-like shape.

The YF-12 swapped the A-12’s Perkins Type I stereo camera system for three GAR-9 missiles. They were stored in three payload bays below and behind the cockpit. The YF-12 performed the first unpowered drop test with a GAR-9 in April 1964, but the test failed: the missile separated, nose up. If a rocket motor had ignited, it would have gone straight into the aircraft cabin. However, less than a year later, a YF-12 scored the first air-to-air kill, shooting down a target 36.2 miles away. In a test in September 1965, a YF-12 launched another GAR-9, from an altitude of 75,000 feet, at a speed of Mach 3.26, achieving another interception at a similar distance.

The YF-12 was always meant to be a technology demonstrator. Now that the technology was proven, engineers put the production version of the aircraft on the drawing board, the fighter-bomber combination, designated the FB-12. Even more ambitious than the YF-12, the FB-12 was meant to carry both the GAR-9 missile, (later known as the AIM-47 Falcon) and the Short Range Attack Missile, or SRAM. The SRAM was an air-to-ground nuclear missile with a range of about 130 miles and a warhead of 17 kilotons – greater than the 15 kiloton yield of the Hiroshima “Little Boy” bomb. SRAM would likely allow the FB-12 to fly deep into Russia, attacking ground targets in concert with larger, nuclear-armed bombers.

The Air Force secured funding for 93 FB-12 fighters, but the planes were never built. Once again, a shift in nuclear strategy left aircraft with no useful role. US military policy no longer called for shooting down enemy bombers, but shifted toward a strategy that deterred the enemy from launching an attack to begin with. Meanwhile, a fighter-bomber armed with short-range nuclear missiles competed in the Air Force budget with the Mach 3-capable XB-70 Valkyrie bomber. The FB-12 was fast, but it couldn’t outrun a changing world. Despite coming tantalizingly close, the US government never built a Mach 3 interceptor.

As time went on, fighter aircraft design reduced speed in favor of maneuverability, weapons capability, aircraft range, and stealth. But you can still see one of the three remaining YF-12As on display at the National Museum of the US Air Force in Dayton, Ohio. NASA leased the other two for research purposes.

The A-12 line left behind a proud line of work in 32 SR-71 Blackbirds – with their muscular capabilities, the Blackbird can be considered a direct descendant of the A-12. The Air Force did get its Mach 3 aircraft eventually, at least until they retired from service in 1999.