Until Lockheed Martin finally builds the SR-72 Mach-6 spy plane it announced in November 2013 — and which reportedly appeared in plain view for the first time in July 2017 — 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.

So to gather intelligence in foreign skies, in 1964 U.S. president Lyndon Johnson announced that the Lockheed Advance Development Projects, also known as the Skunk Works, would build another strategic reconnaissance aircraft—one so fast that no other airplane could reach it.

The Blackbird’s flight characteristics were incredible. It was able to fly at more than three and a half times the speed of sound at 88,000 feet. To give you an idea of what that altitude means, the SR-71 took photos from three times the height of Mount Everest and its pilots dressed in full pressure suits like astronauts.

During its more than three-decade career, which ended on Oct. 9, 1999, no SR-71 was lost to hostile action. But not for a lack of trying on the part of America’s enemies. Neither enemy fighters nor surface-to-air missiles were ever able to shoot down or damage an SR-71.

High speed was one factor. But the Blackbird was also hard to detected by radar, being the first aircraft to feature stealth technology. A special paint was used on the SR-71’s wings, tail and fuselage. Containing iron ferrite, this paint absorbed radar energy instead of returning it to the sender.

With a Radar Cross Section equivalent to a small light aircraft, the SR-71 appeared on enemy radar screens too late for a missile computer to estimate its direction for a successful shoot-down. The Blackbird also masked its range and bearing by jamming enemy listening devices with sophisticated electronic countermeasures.

Not only were SAMs unable to catch the Blackbird, even the the fastest Soviet fighter jets lacked the necessary speed to intercept the SR-71. Soviet pilot Viktor Belenko, who defected to Japan in a MiG-25 on Dec. 6, 1976, confirmed this in his book MiG Pilot.

“American reconnaissance planes, SR-71s, were prowling off the coast, staying outside Soviet airspace by photographing terrain hundreds of miles inland with side-angle cameras,” Belenko wrote. “They taunted and toyed with the MiG-25s sent up to intercept them, scooting up to altitudes the Soviet planes could not reach, and circling leisurely above them or dashing off at speeds the Russians could not match.”

Continued Belenko:

“[The Soviets] had a master plan to intercept an SR-71 by positioning a MiG-25 in front of it and one below it, and when the SR-71 passed they would fire missiles. But it never occurred. Soviet computers were very primitive, and there is no way that mission can be accomplished.

First of all, the SR-71 flies too high and too fast. The MiG-25 cannot reach it or catch it. Secondly … the missiles are useless above 27,000 meters [88,000 feet], and as you know, the SR-71 cruises much higher. But even if we could reach it, our missiles lack the velocity to overtake the SR-71 if they are fired in a tail chase. And if they are fired head-on, the guidance systems cannot adjust quickly enough to the high closing speed.”

Moreover, Belenko’s missiles would have not worked because “most air- to-air missiles are optimized to maneuver in the thicker air below around 30,000 feet in order to shoot down an enemy plane,” former Blackbird pilot Col. Richard Graham explained in his book SR-71: The Complete Illustrated History of the Blackbird. “Firing at the SR-71, cruising at 75,000 feet, the air is so thin that any maneuvering capability of the missile is practically nonexistent.”

With the Mach-6 SR-72 a real possibility, “speed is the new stealth” is Lockheed Martin’s new slogan. But really, there’s nothing new about it. Speed has worked to protect spy planes for the last 60 years.