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It flew 135 times, all in daylight, over a three-year span in the 1980s, and yet no one apparently ever spotted it. "Tacit Blue," as the experimental plane was known, turned out to be one of the military's best-kept secrets.

More than 11 years after Tacit Blue made its last test flight, the Air Force declassified it on Tuesday, complete with color photos and a videotape of the airplane that in a side view looks like an upside-down bathtub with wings.The idea behind Tacit Blue - to build a spy plane that could not be detected on radar - died in 1985 after the Air Force spent $165 million to manufacture and test the first and only prototype. But the secrecy lived on while the "stealth" technologies were adapted for use on today's B-2 bomber.

The plane has been in storage since 1985. It is scheduled to go on permanent public display May 22 at a museum at Wright-Patterson Air Force Base, Ohio.

The Air Force had never before acknowledged the existence of the project, which was managed at a level of secrecy - "special access required" - that is higher than top secret.

The plane was built between 1978 and 1982 by Northrop Corp. at its Hawthorne, Calif., plant for $136 million, Lt. Gen. George Muellner told a Pentagon news conference. Testing the plane cost about $29 million more, he said.

"It has been a pretty well-kept secret," Muellner said.

Aircraft enthusiasts have speculated for years about the existence of a super-secret spy plane some dubbed "Aurora," but that plane - which U.S. officials deny ever existed - was supposed to be supersonic. Tacit Blue was subsonic.

John Pike, an aviation specialist at the Federation of American Scientists, said none of the private groups that hunt for clues to secret military programs had ever spotted Tacit Blue or speculated on its existence. Pike said the full story of Tacit Blue and other secret aircraft is yet to be told.

"I would be surprised if there weren't several more" like this yet to be declassified, Pike said in an interview.

Shaped like no other known military aircraft, Tacit Blue is 55 feet long with a 48-foot wingspan. That makes it roughly the size of an Air Force fighter jet.

The intake for Tacit Blue's two turbofan engines is on top of the fuselage. The vertical stabilizers on the rear of the plane form a "V." The plane's underside appears to form an unbroken flat surface from front to rear.

Two aspects of Tacit Blue's innovative radar-evading characteristics were adapted for use on the B-2 bomber, Air Force officials said. One is the combination of curved and linear surfaces; the other is the special composite materials used on the surfaces to absorb signals from radars trying to track it.