Launched in 1946 with the Bell X-1, the US X-Plane program is a series of aircraft, missiles and rockets built solely to develop advanced aerospace technology rather than production aircraft. Essentially, they are proof-of-concept platforms made in very small numbers that tolerate a higher risk factor than standard aerospace development.

Normally, the numbers assigned to X-planes are sequential, though exceptions are sometimes made for symbolic reasons. This is the case with the X-76, which was named in celebration of the upcoming 250th anniversary of the United States in 2026.

Up until now, the X-76 has been in its conceptual and design phase, but with the new official designation it's moving on to Phase 2, which handles detailed design and fabrication, followed by the Phase 3 flight test program beginning in 2027. Its purpose is to bridge the gap between rotorcraft like the V-22 Osprey and high-performance jets.

In other words, the goal is to create an aircraft that can take off and land from bare fields like a helicopter, yet boasts the speed and performance of a jet. This is important because most jets require a runway to operate, while rotorcraft have very limited flight performance. The X-76 aims to remove these limitations and combine the advantages of both.

The basic problem with rotorcraft like the V-22 Osprey is that the large rotors providing lift and propulsion are also an inherent limiting factor. At high speeds, the rotors generate significant drag, which worsens as the blades spin faster. That means the Osprey can only reach a top speed of about 270 knots (311 mph, 500 km/h).

Ideally, the best solution, according to DARPA, is to get rid of the rotors when the aircraft transitions into high-speed horizontal flight. The X-76 does this by stopping the rotors and folding them back against the nacelle body when the engine converts to a full turbojet configuration. That sounds simple, but it requires eliminating the complex gearbox of the Osprey and replacing it with a new configuration that incorporates two distinct thermodynamic cycles – one for static lift and one for high-subsonic cruising.

While hovering or during vertical takeoff, the X-76 operates like a conventional tilt-rotor. In this mode, the high-performance turbine core engine drives a shaft connected to the rotors. As the aircraft accelerates forward, the wings begin generating lift. When a critical speed of about 150 to 200 knots (173 to 230 mph, 278 to 370 km/h) is reached, the clutch to the rotors disengages and the rotors feather as they fold aft to form a streamlined aerodynamic pod.