Boeing X-66

The Boeing X-66 was an experimental airliner under development by Boeing as part of the X-plane series created with NASA under the Sustainable Flight Demonstrator programme. It used an extra-long, thin wing stabilised by diagonal bracing struts, known as a Transonic Truss-Braced Wing, based on the Subsonic Ultra-Green Aircraft Reach research studies that examined truss bracing and hybrid electric technology. Wind tunnel work at NASA Ames and Langley led to a 170 ft span wing that folds outboard of the truss so it can use 737-size airport gates, and compared with cantilever wings it promised an 8 to 10 percent fuel burn reduction. The full-scale demonstrator was to be a rebuilt shortened McDonnell Douglas MD-90 fitted with a dedicated Pratt & Whitney geared turbofan, funded by NASA and Boeing, but Boeing paused the programme indefinitely in April 2025.