The Boeing X-66 is an experimental airliner under development by Boeing. It is part of the X-plane series as the X-66, and has been developed in collaboration with NASA and its Sustainable Flight Demonstrator program. It will use extra-long, thin wings stabilized by diagonal struts, which is known as a Transonic Truss-Braced Wing concept. The aircraft configuration is based on research studies referred to as "Subsonic Ultra-Green Aircraft Reach (SUGAR)" which extensively studied truss-bracing and hybrid electric technologies. 
By early 2019, following extensive wind tunnel testing at NASA Ames Research Center, an optimized truss and more sweep for the 170 ft (52 m) span wing allowed flying higher and faster, up from Mach 0.70–0.75 to Mach 0.80 like current jetliners. Compared to aircraft with cantilevered wings, fuel burn should be reduced by 8-10%, and the technology should be ready in 2030-2035. Aspect ratios up to 27 were evaluated, up from 8-10 for current narrowbodies. The design was presented at the January 2019 AIAA conference and the wing folds outboard of the truss to use airport gates for the 118 ft (36 m)-span 737. (ICAO aerodrome code C)
The wing has 20° of sweep and was moved forward, the truss section was optimised, tapered with an increased root chord, has a trailing edge with forward sweep and generates lift. It should be tested from early 2019 at the NASA Ames 11 ft (3.4 m) transonic tunnel, then later in 2019 at the 14 by 22 ft (4.3 by 6.7 m) subsonic tunnel at NASA Langley. A full-scale X-plane could be developed and tested under NASA's UEST plan, outlined in the New Aviation Horizons flight demonstration from 2023. Boeing proposed to modify an MD-80 powered by turbofans, but it could test a series/parallel hybrid electric propulsor based on Rolls-Royce LibertyWorks' EVE concept of a geared turbofan. An 1.5 MW (2,000 hp) electric motor/generator mounted between the compressor and the variable pitch fan, fed by batteries to boost the takeoff and climb, allows for a smaller engine and improves efficiency by 4.5% over a 3,500 nmi (6,500 km) mission. A tail-mounted BLI fan would ingest and reenergize the slow airflow over the fuselage.
A full-scale demonstrator was announced on January 18, 2023, with NASA funding $425 million over seven years while Boeing and its partners will invest the remainder of the agreed $725 million, as the airframer had already spent $110 million in sustainable aviation research. Combined with better propulsion and materials, efficiency is targeted at 30% better than current Boeing 737 MAX and Airbus A320neo. Using a shortened MD-90 airframe and CFM International RISE engines, the demonstrator is scheduled to fly in 2028, providing the basis for a possible family of 130-to-210-seat aircraft. The full-scale Sustainable Flight Demonstrator (SFD) is designated the X-66A.