The X-Planes, Ranked: 12 Experimental Aircraft That Changed Flight

The most important X-plane ever flown is the North American X-15. Across 199 flights between 1959 and 1968 it reached Mach 6.7 and 354,200 feet, and NASA says its data fed Mercury, Gemini, Apollo and the Space Shuttle.

X-planes are the United States’ experimental research aircraft, the ones that carry an “X” designation because their only job is to test an idea. We rank the 12 that mattered most by impact: what each one proved first, and how far that knowledge traveled into aircraft and spacecraft that actually went into service.

The Bell X-1, the first aircraft to fly faster than sound in level flight, takes second. The full list of every X designation, from X-1 to the X-76 announced in March 2026, sits further down. Everything here is current as of October 2026.

Rank	X-plane	Defining moment	Status (October 2026)
1	North American X-15	Mach 6.7 and 354,200 ft	Retired 1968
2	Bell X-1	First supersonic level flight, 1947	Retired
3	Lockheed Martin X-35	Short takeoff, supersonic dash and vertical landing in one flight, 2001	Became the F-35
4	Boeing X-37B	908 days in orbit, 2020 to 2022	In orbit on its 8th mission
5	Lockheed Martin X-59	Mach 1.4 at 55,000 ft, 2026	In flight test
6	NASA X-43A	Mach 9.6 on scramjet power, 2004	Program complete
7	Bell X-5	Wings swept in flight, 1951	Retired
8	Martin Marietta X-24B	Precise unpowered runway landings	Last powered flight 1975
9	Northrop Grumman X-47B	Drone arrested landing on a carrier, 2013	Retired
10	Grumman X-29	Forward-swept wing, first flown 1984	Retired
11	Rockwell-MBB X-31	Controlled flight at 70 degrees angle of attack, 1992	Retired 2003
12	Bell X-2	First flight past Mach 3, 1956	Retired

1. North American X-15

North American built three X-15s, and between them they flew 199 times with 12 pilots. Scott Crossfield made the first glide flight on June 8, 1959, and the first powered flight on September 17, 1959. Bill Dana flew the last one on October 24, 1968.

Air Force pilot Pete Knight took the modified X-15A-2 to 4,520 mph (7,274 km/h), or Mach 6.7, on October 3, 1967.

Four years earlier, on August 22, 1963, NASA pilot Joe Walker climbed to 354,200 feet, about 67 miles (108 km) up. That speed still puts it near the top of our ranking of the fastest aircraft in the world.

It beats the X-1 for first place because of what came after. The program produced more than 765 research reports, and NASA credits it with data used for Mercury, Gemini, Apollo, the Saturn rockets and the Space Shuttle.

It also had a cost. Michael Adams was killed on November 15, 1967, when his X-15 entered a spin he could not recover from after a failure in the adaptive flight control system.

2. Bell X-1

On October 14, 1947, Air Force Captain Chuck Yeager flew the X-1 “Glamorous Glennis” to Mach 1.06, about 700 mph (1,127 km/h), at 43,000 feet. It was the first time a pilot had flown faster than sound, and it was done after a drop from a B-29 bomber.

Yeager was not its first pilot. Jack Woolams made the first glide flight on January 19, 1946, and Bell pilot Chalmers “Slick” Goodlin made the first powered flight in December 1946. Bell built three of the rocket planes, later followed by the X-1A, X-1B and X-1D.

The X-1 settled whether the “sound barrier” was a wall, which makes it the most famous X-plane. It ranks second only because the X-15 carried the program much further, all the way to the edge of space.

3. Lockheed Martin X-35

Most X-planes are never meant to become production aircraft. The X-35 is the clearest exception: it was Lockheed Martin’s entry in the Joint Strike Fighter contest, and it became the F-35 Lightning II.

The X-35A first flew on October 24, 2000, about five weeks after Boeing’s rival X-32. The decisive moment came on July 20, 2001, when the X-35B made a 450-foot short takeoff, climbed to 25,000 feet, dashed supersonic and came back for a vertical landing, an aviation first.

Lockheed Martin won the $18.9 billion development contract on October 26, 2001. No other X-plane led so directly to a front-line aircraft, which is why it outranks the record-breakers below it.

4. Boeing X-37B

The X-37B is the only X-plane on this list that flies in orbit. Boeing built two of the uncrewed spaceplanes, and the first launched on an Atlas V on April 22, 2010.

Each mission has stayed up longer than the last, until the seventh. The sixth mission spent 908 days in orbit before landing at Kennedy Space Center on November 12, 2022, and the seventh returned on March 7, 2025, after more than 434 days.

The eighth mission launched on a Falcon 9 on August 21, 2025 (Eastern time), and the Space Force had announced no landing as of early October 2026. What it carries is largely secret, which is part of why it draws so many searches.

5. Lockheed Martin X-59

The X-59 is built to make a sonic boom quiet enough that supersonic flight over land could be allowed again. Lockheed Martin gives it a 99 ft 7 in (30.4 m) length, a single General Electric F414 engine and a design target below 75 PLdB, a measure of perceived loudness.

It first flew on October 28, 2025, and went supersonic for the first time on June 5, 2026. A week later it reached its design point of Mach 1.4, about 925 mph (1,489 km/h), at 55,000 feet, and by early October 2026 it had flown 38 times and touched Mach 1.53.

Its mission has been overtaken by politics. An executive order of June 6, 2025, told the FAA to repeal the 1973 overland supersonic ban, and the agency proposed a rule built on a sonic-boom limit in July 2026. The X-59’s community overflights, meant to show what people actually hear, have not started yet.

6. NASA X-43A

The X-43A was an uncrewed, 12-foot scramjet test vehicle, carried aloft by a B-52B and boosted by a modified Pegasus rocket. The first attempt failed in June 2001 when the booster went out of control.

The next two worked. It hit Mach 6.8 on March 27, 2004, and Mach 9.6, nearly 7,000 mph (11,300 km/h), on November 16, 2004, with the scramjet burning for about 10 seconds each time. Guinness World Records lists it as the fastest aircraft with an air-breathing engine.

Its successor, the Boeing X-51A Waverider, traded peak speed for duration: 210 seconds of scramjet flight at Mach 5.1 on May 1, 2013. Both feed today’s push for hypersonic aircraft.

7. Bell X-5

The X-5 first flew on June 20, 1951, and its wings could pivot from 20 to 60 degrees of sweep in flight, a change that took about 20 seconds. Its predecessor design could only have its sweep adjusted on the ground.

NASA says the X-5’s sliding mechanism was not practical, and a later Langley design moved the pivots outboard. That fix produced the swing-wing generation: the F-111, F-14, B-1, Tornado, Tu-160 and others.

Two were built, and one was lost. Major Raymond Popson was killed when the second X-5 entered a spin on October 13, 1953.

8. Martin Marietta X-24B

The X-24B was a wingless lifting body, rebuilt from the earlier X-24A, that got its lift from the shape of its fuselage. John Manke flew its first glide on August 1, 1973, and Mike Love took it to 1,164 mph (1,873 km/h) on October 25, 1974.

Its biggest contribution was a landing. Among its final flights were two precise, unpowered landings on the main concrete runway at Edwards, proof that a returning spacecraft could glide to a runway.

NASA says the lifting body program helped write the flight plan for Space Shuttle landings, down to the approach pattern and landing speed.

9. Northrop Grumman X-47B

The X-47B was the first unmanned aircraft to make an arrested landing aboard a modern aircraft carrier, catching the 3 wire on USS George H.W. Bush on July 10, 2013. It had been catapulted off the same ship two months earlier, on May 14.

On April 22, 2015, it became the first unmanned aircraft to refuel autonomously in flight, taking on more than 4,000 pounds (1,800 kg) of fuel from a tanker. For where unmanned combat aircraft stand today, see our ranking of the best military drones in the world.

10. Grumman X-29

The Grumman X-29 flew its wings backward: a 30-degree forward sweep that cut transonic drag by as much as 20 percent. The first of two aircraft flew on December 14, 1984.

The shape was so unstable that a triple-redundant digital fly-by-wire system issued up to 40 corrections a second to keep it flying. The second X-29 reached 67 degrees angle of attack, with excellent control up to 45 degrees.

Forward-swept wings never made it into a production fighter. What the X-29 really proved was that computers could keep a deliberately unstable aircraft flying, a principle modern fighters rely on.

11. Rockwell-MBB X-31

The X-31 was a US-German project, with two aircraft built to test thrust vectoring for close-in dogfighting. Ship 1 first flew on October 11, 1990, and the second X-31 reached controlled flight at 70 degrees angle of attack on November 6, 1992.

In simulated combat, NASA records a potential kill ratio of 30 to 1. The program flew 555 times before Ship 1 was lost on January 19, 1995, when ice in an unheated pitot probe fed bad data to the flight computers; the pilot ejected safely.

The surviving jet returned as the Navy’s VECTOR program and made the last X-31 flight on April 29, 2003.

12. Bell X-2

On September 27, 1956, Milburn Apt flew the X-2 to 2,094 mph (3,370 km/h), Mach 3.196, the first flight past three times the speed of sound. The aircraft then tumbled out of control from inertia coupling, and Apt was killed when his escape capsule hit the ground.

Weeks earlier, Iven Kincheloe had taken it to 126,200 feet, a record that stood until the X-15. It ranks 12th because the X-15 soon passed both of its records.

The X-Planes Still to Come

Three new X-planes are being built as of October 2026. The DARPA and Aurora X-65 CRANE steers with jets of air instead of moving control surfaces; its wings were delivered in June 2026, and first flight has slipped to 2027.

General Atomics’ X-68A LongShot, designated on February 17, 2026, is an air-launched drone that carries air-to-air missiles, with a first launch from an F-15 expected around the end of 2026.

Bell’s X-76 SPRINT, announced on March 9, 2026, stops and folds its rotors in flight to cruise at 400 to 450 knots, with flight tests planned for early 2028.

The Boeing and NASA X-66, a truss-braced wing airliner demonstrator, was paused in April 2025 in favor of ground testing.

The X-62A VISTA, a heavily modified F-16, has become the Air Force’s AI test jet: in 2023 an AI flew it against a human-piloted F-16, and Air Force Secretary Frank Kendall rode in it under AI control in May 2024.

The X-Planes That Never Flew

Not every X number became an aircraft. Defense Secretary Robert McNamara canceled the X-20 Dyna-Soar spaceplane on December 10, 1963, and Congress ended funding for the X-30 National Aerospace Plane in 1994.

NASA stopped funding the X-33 on March 1, 2001, after spending $912 million on a vehicle that never flew. The all-electric X-57 Maxwell ended the same way in 2023, never leaving the ground.

Every X-Plane Ever Designated

The series runs from X-1 to X-68, plus the X-76, whose number nods to 1776 for the country’s 250th anniversary.

Several of the early numbers went to missiles and rockets, X-52 was skipped to avoid confusion with the B-52, and the X-58 and X-67 slots went to the XQ-58A and XQ-67A drones.

Designation	Builder	What it tested
X-1 (and X-1A to X-1E)	Bell	Supersonic and high-altitude flight
X-2 Starbuster	Bell	Mach 3 and high-altitude flight
X-3 Stiletto	Douglas	Trapezoidal wing, titanium structure
X-4 Bantam	Northrop	Tailless transonic flight
X-5	Bell	Variable wing sweep in flight
X-6	Convair	Nuclear propulsion (never built)
X-7	Lockheed	Ramjet test missile
X-8 Aerobee	Aerojet	Upper-atmosphere research rocket
X-9 Shrike	Bell	Guided missile technology
X-10	North American	Navaho cruise missile testbed
X-11	Convair	Atlas missile testbed
X-12	Convair	Atlas missile testbed
X-13 Vertijet	Ryan	Jet-powered tail-sitting VTOL
X-14	Bell	Vectored-thrust VTOL
X-15 (and X-15A-2)	North American	Hypersonic and high-altitude flight
X-16	Bell	High-altitude reconnaissance (never flew)
X-17	Lockheed	Reentry research missile
X-18	Hiller	Tilt-wing V/STOL
X-19	Curtiss-Wright	Tandem tiltrotor VTOL
X-20 Dyna-Soar	Boeing	Military spaceplane (canceled 1963)
X-21A	Northrop	Boundary-layer control
X-22	Bell	Ducted-fan V/STOL
X-23 PRIME	Martin Marietta	Maneuvering reentry
X-24A and X-24B	Martin Marietta	Lifting bodies
X-25	Bensen	Autogyro for downed pilots
X-26 Frigate	Schweizer	Training glider and quiet observation
X-27	Lockheed	Fighter research (never flew)
X-28 Sea Skimmer	Osprey	Low-cost patrol seaplane
X-29	Grumman	Forward-swept wing
X-30 NASP	Rockwell	Single-stage-to-orbit spaceplane (canceled)
X-31	Rockwell-MBB	Thrust-vectoring supermaneuverability
X-32	Boeing	Joint Strike Fighter demonstrator
X-33	Lockheed Martin	Reusable launch vehicle (never flew)
X-34	Orbital Sciences	Uncrewed reusable spaceplane (never flew)
X-35	Lockheed Martin	Joint Strike Fighter demonstrator
X-36	McDonnell Douglas	Scale tailless fighter
X-37	Boeing	Reusable orbital spaceplane
X-38	Scaled Composites	Space station crew return vehicle
X-39	Not disclosed	Future aircraft technology
X-40A	Boeing	Scale X-37 prototype
X-41	Not disclosed	Maneuvering reentry vehicle
X-42	Orbital Sciences	Upper-stage rocket (never flew)
X-43A Hyper-X	Micro-Craft	Hypersonic scramjet
X-44 MANTA	Lockheed Martin	Tailless thrust vectoring (never flew)
X-45	Boeing	Unmanned combat air vehicle
X-46	Boeing	Naval unmanned combat air vehicle (never flew)
X-47A and X-47B	Northrop Grumman	Carrier-based unmanned combat air vehicle
X-48	Boeing	Blended wing body
X-49 SpeedHawk	Piasecki	Compound helicopter
X-50 Dragonfly	Boeing	Canard rotor/wing
X-51 Waverider	Boeing	Hypersonic scramjet
X-52	None	Skipped
X-53	Boeing	Active aeroelastic wing
X-54	Gulfstream	Low-boom supersonic research
X-55	Lockheed Martin	Composite cargo aircraft
X-56	Lockheed Martin	Flutter suppression
X-57 Maxwell	ESAero	All-electric propulsion (never flew)
X-58	Kratos	Assigned as the XQ-58A Valkyrie drone
X-59	Lockheed Martin	Quiet supersonic flight
X-60	Generation Orbit	Air-launched hypersonic rocket
X-61 Gremlins	Dynetics	Air-launched, air-recovered drone
X-62 VISTA	Lockheed Martin and Calspan	In-flight simulator and AI flight
X-63	ABL Space Systems	Launch vehicle testbed
X-64	Invocon	Launch vehicle testbed
X-65 CRANE	Aurora Flight Sciences	Active flow control
X-66	Boeing	Transonic truss-braced wing
X-67	General Atomics	Assigned as the XQ-67A drone
X-68 LongShot	General Atomics	Air-launched missile-carrying drone
X-76 SPRINT	Bell	High-speed stop-fold rotor VTOL

Some of the less famous numbers did serious work too. The remotely piloted Boeing X-48B, for one, flew 92 times between 2007 and 2010 to test the blended wing body shape for Boeing, NASA and the Air Force Research Laboratory.

FAQ

What are X-planes?

X-planes are experimental US aircraft and research vehicles that carry an X designation because their job is to test new technology, not to fly operational missions. The series began with the Bell X-1 in the 1940s and has been sponsored by NASA and its predecessor NACA, the Air Force, the Navy and DARPA.

What was the first X-plane?

The first X-plane was the Bell X-1. Its first glide flight came on January 19, 1946, and on October 14, 1947, Chuck Yeager flew it to Mach 1.06, the first time a pilot flew faster than sound.

What is the fastest X-plane?

The fastest X-plane is the uncrewed NASA X-43A, which reached Mach 9.6, nearly 7,000 mph, on November 16, 2004. The fastest crewed X-plane is the X-15, which Pete Knight flew to 4,520 mph, or Mach 6.7, on October 3, 1967.

How many X-planes are there?

Designations run from X-1 to X-68, plus the X-76 announced in March 2026. Not every number is an airplane: several early ones were missiles and rockets, some were never built or never flew, X-52 was skipped, and the X-58 and X-67 numbers went to the XQ-58A and XQ-67A drones.

Did any X-plane go into production?

Yes. The Lockheed Martin X-35, which first flew on October 24, 2000, won the Joint Strike Fighter contest in October 2001 and became the F-35 Lightning II. Most X-planes are built in ones and twos and never lead directly to a production aircraft.

What is the newest X-plane?

The newest designation is the Bell X-76 SPRINT, announced by DARPA and Bell on March 9, 2026. It is designed to stop and fold its rotors in flight and cruise at 400 to 450 knots, with flight testing planned for early 2028.

Has the X-59 flown supersonic yet?

Yes. The X-59 first flew on October 28, 2025, broke the sound barrier on June 5, 2026, and reached its design point of Mach 1.4 at 55,000 feet a week later. Its community overflights, which test how quiet its boom sounds on the ground, had not started as of early October 2026.

Is X-Plane the flight simulator related to the X-planes?

No. X-Plane is a commercial flight simulator that shares the name. The real X-planes are US government research aircraft, from the Bell X-1 to the X-76.