Boeing X-66
Experimental · USA
The Boeing X-66 is a NASA Sustainable Flight Demonstrator (SFD) being built by Boeing on a modified McDonnell Douglas MD-90 airframe. The X-66 is the largest crewed X-plane in active development as of 2026. The aircraft will test a Transonic Truss-Braced Wing (TTBW) configuration — a much higher-aspect-ratio wing than a conventional airliner, supported mid-span by a strut bracing it to the lower fuselage. The TTBW promises 10-20% fuel-burn reduction at typical airliner cruise speeds. First flight is targeted for 2028.
The aerodynamic logic is straightforward. Higher-aspect-ratio wings (long, narrow) have lower induced drag than short stubby wings — a 30% increase in span typically buys 10-15% lower drag at cruise. Conventional airliners can't simply make their wings longer because the bending loads on a thin long wing exceed material limits. The truss-braced configuration solves this by adding a structural strut from mid-wing to the lower fuselage, like the diagonal struts on a wooden bridge. The strut takes most of the bending load; the wing itself can then be thinner, longer, and more efficient.
Boeing took an existing MD-90 airframe — a 1990s twin-jet airliner with about 150 seat capacity — and is rebuilding it with a brand-new TTBW wing pair. The MD-90's existing fuselage, tail, and engines stay; the wings are completely replaced with a longer (170 ft / 51.8 m vs. MD-90's 108 ft / 32.9 m), higher-aspect-ratio design with mid-span struts. New fly-by-wire flight control software handles the active flutter suppression that the long flexible wing will need (drawing on data from the Lockheed X-56 MUTT).
The X-66 was designated in June 2023, with NASA contributing USD$425 million and Boeing contributing approximately USD$725 million toward the seven-year development. The aircraft is being assembled at Boeing's Palmdale plant in California from 2024 onward; first flight is scheduled for 2028 from Edwards AFB. If the TTBW configuration validates in flight test, Boeing has indicated it could be the basis for a successor to the 737 family — a transformative airliner architecture that has not changed substantially since the early 1960s.
For Kids — a shorter, friendlier version
The Boeing X-66 is a special test airplane. NASA and Boeing are building it together. It is based on an older jet called the MD-90. The X-66 is the largest crewed X-plane being built right now.
The X-66 has a very special wing design. The wings are long and thin. A strong strut holds each wing up in the middle. This strut works like the diagonal beams on a wooden bridge. It keeps the long wing from bending too much.
These long, thin wings are faster than short stubby wings at cutting through the air. They create less drag, which means the engine does not have to work as hard. This new wing design could save between 10 and 20 percent of the fuel a normal airliner uses.
The X-66 is longer than a typical school bus by a wide margin. Boeing took an old MD-90 jet and changed it into this exciting new test plane. The first flight is planned for the year 2028.
Fun Facts
- The X-66 is the largest crewed X-plane being actively built as of 2026.
- Its special wings are longer than a normal airliner's wings by about 30 percent.
- A strut under each wing works like a bridge beam to hold the long wing steady.
- The new wing design could cut fuel use by up to 20 percent on every flight.
- Boeing used an old MD-90 passenger jet as the starting point for the X-66.
- Long, thin wings create less drag, making flight more efficient at cruising speed.
- The first flight of the X-66 is targeted for 2028.
Kids’ Questions
Why does the X-66 have such long wings?
Long, thin wings push through the air more easily than short, wide ones. They create less drag, so the plane uses less fuel. The special strut stops the long wing from bending or breaking.
What is the strut on the X-66 wing for?
The strut is a strong support that runs from the middle of the wing down to the bottom of the plane's body. It holds the long wing steady, just like diagonal beams hold up a bridge. Without it, such a long thin wing would bend too much.
When will the X-66 fly for the first time?
The first flight is planned for 2028. Engineers are still building and testing the plane. It will help show if the new wing design can save fuel on real airliners one day.
Variants
- X-66 (in development)
- Single airframe under construction at Boeing Palmdale, California. MD-90 fuselage with new 170-ft Transonic Truss-Braced Wing pair. First flight targeted 2028, programme runs through 2030.
Notable Operators
- NASA
- Programme sponsor. NASA contributed USD$425 million to the seven-year development. Programme management at NASA's Langley Research Center; flight testing will be at NASA Armstrong, Edwards AFB.
- Boeing Phantom Works
- Designer and builder. Boeing contributed approximately USD$725 million in cost-share funding. Assembly at Boeing Palmdale, California.
Frequently Asked Questions
What is a Transonic Truss-Braced Wing?
A wing configuration that uses a structural strut bracing the wing mid-span to the lower fuselage, like the diagonal struts on a wooden bridge. The strut takes the bending load, allowing the wing itself to be thinner, longer, and more aerodynamically efficient. At airliner cruise (Mach 0.78-0.82) the configuration promises 10-20% fuel-burn reduction.
When will the X-66 fly?
First flight is targeted for 2028. Assembly at Boeing's Palmdale plant began in 2024; flight testing will run through 2030 from Edwards AFB.
Will the TTBW concept reach production?
Boeing has indicated that, if the X-66 flight-test programme validates the TTBW configuration, it could be the basis for a successor to the 737 family. No firm production programme has been announced; the 737 successor decision is expected in the late 2020s.
What aircraft is the X-66 based on?
An existing McDonnell Douglas MD-90 airliner. The MD-90's fuselage, tail, and engines are retained; the original wings are completely replaced with the new TTBW wing pair.
How does the X-66 manage flutter?
The long flexible TTBW wing is at risk of aeroelastic flutter — a divergent oscillation that can break the wing. The X-66 uses an active flutter-suppression flight-control system, drawing on data from NASA's earlier Lockheed X-56 MUTT programme.
Sources
See Also
- Lockheed X-56 MUTT — Earlier active flutter-suppression UAV
- Boeing X-48 BWB — Sister Boeing airliner-technology demonstrator
- NASA X-57 Maxwell — Sister NASA airliner-technology demonstrator (cancelled)
- Boeing 787 Dreamliner — Production airliner that TTBW would compete with
- Grumman X-29 — Earlier flexible-wing X-plane