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12/31/2007. This was the first of two WB-66Ds, that were modified by Northrop to the X-21A laminar flow control research aircraft. A new wing was fitted and the two 9,700 lb (4,400 kg) s.t Allison J71-A-13 engines slung from the wing were replaced by two 9,490 lb (4,305 kg) s.t General Electric XJ79-GE-13 engines fitted at the rear of the fuselage. In each of the underwing fairings was a Garrett/AiResearch blade-compressor that sucked the boundary layer into the wing through a series of fine spanwise slots and exhausted the air rearward to obtain additional thrust. The first X-21A was flown for the first time on April 18, 1963, followed by the second, s/n 55-0410 c/n 45042, in August 1963. Both aircraft are in existence at Edwards AFB, California.