The Northrop X-21A was an experimental aircraft designed to test wings with laminar flow control. It was based on the Douglas WB-66D airframe, with the wing-mounted engines moved to the rear fuselage and making space for air compressors. The aircraft first flew on 18 April 1963 with NASA test pilot Jack Wells at the controls. Although useful testing was accomplished, the extensive maintenance of the intricate laminar-flow system caused the end of the program.
In initial testing there were significant problems with the porous materials and surface slots getting plugged with debris, bugs, even rain. In certain conditions, ice crystals would form due to the rapid cooling of air over those laminar surfaces abruptly disrupting laminar flow, causing rapid melting and rapid transition back to turbulent flow. Maximum achievement of 95 percent laminar flow over those areas was desired. However, the design effort was canceled due to the plugging problems.
Nevertheless, pioneering data was obtained in the X-21 flight program, including the effects of surface irregularities, boundary-layer turbulence induced by three-dimensional span-wise flow effects in the boundary layer (referred to as span-wise contamination) and degrading environmental effects such as ice crystals in the atmosphere.
Both X-21As ended up in storage at Edwards Air Force Base, California where gradually, they became derelicts, used primarily as photo targets. The remains can still be viewed but no efforts have been made to recover a single example for restoration or display.
http://en.wikipedia.org/wiki/Northrop_X-21