Bell’s Stop-Fold Tiltrotor High-Speed VTOL Becomes X-76
The stop-fold tiltrotor uncrewed aircraft under development by Bell for a DARPA project to fly a high-speed vertical-takeoff-and-landing (VTOL) demonstrator has been designated the X-76.
Under Phase 2 of DARPA’s Speed and Runway Independent Technologies (Sprint) project, Bell has completed the critical design review (CDR) and moved into manufacturing of the demonstrator.
Bell’s stop-fold tiltrotor was selected over a fan-in-wing design from Boeing’s Aurora Flight Sciences in May 2025. Phase 2 is to culminate in ground testing of the demonstrator, with flight testing expected in early 2028 under a planned third phase of the Sprint program.
Sprint is intended to demonstrate an aircraft that can take off and land vertically from unprepared surfaces, hover like a helicopter and achieve cruise speeds exceeding 400 kt. (460.3 mph), DARPA says. The Bell Boeing V-22 Osprey tiltrotor has a maximum speed of 275 kt.
Bell’s X-76 takes off, hovers and lands like the V-22, with the proprotor tilted vertically and the nacelles tilted down to transition into forward flight. But, as the aircraft accelerates, the proprotors are slowed, stopped and folded back along the nacelles, with propulsion transitioning from turboshaft to turbofan mode.
Bell demonstrated the rotor stop-fold and propulsion transition on the ground in 2023 using a sled on a test track at Holloman AFB, New Mexico. The X-76 is an uncrewed vehicle to validate the technology in flight, but Bell has shown concept images of crewed designs as large as the Lockheed Martin C-130.
The Sprint program is “building options,” DARPA program manager Cmdr. Ian Higgins said in a statement. “We’re working to deliver the option of surprise, the option of rapid reinforcement, and the option of life-saving speed, anywhere on the globe, without needing a runway.”
The last X-plane designation to be assigned was X-68, announced in February for the air-launched, missile-carrying uncrewed aircraft under development by General Atomics Aeronautical Systems for DARPA’s LongShot program.
The out-of-sequence X-76 designation was chosen as “a deliberate nod to the revolutionary spirit of 1776” to coincide with America’s 250th anniversary, DARPA says. This follows the precedent set by the U.S. Army in 2025 when it chose to designate the Future Long Range Assault Aircraft advanced tiltrotor under development by Bell the MV-75 to commemorate the service’s founding in 1775.
