DARPA SPRINT: Bell Wins X-Plane, and the Race to Reinvent VTOL
Bell Textron won the DARPA SPRINT Phase 2 contract in July 2025 with a stop/fold tiltrotor design — rotors that fold mid-flight for jet-speed cruise at 400–450 knots. Aurora was eliminated. The X-plane demonstrator targets first flight in 2028. FY2026 budget: US$55.2 million.
Bell Textron will build the SPRINT X-plane. DARPA announced the Phase 2 down-select on 9 July 2025, awarding Bell the contract to design, fabricate, and flight-test an unmanned demonstrator for the Speed and Runway Independent Technologies programme — a joint DARPA/US Special Operations Command effort to develop high-speed vertical takeoff and landing aircraft. Aurora Flight Sciences, which had competed with a blended-wing-body fan-in-wing concept, was eliminated.
The original version of this article, published in October 2024, covered the Phase 1B contract awards to both Aurora and Bell. The programme has since moved decisively. What follows is a substantially revised account.
1. Bell's Stop/Fold Architecture
Bell's design uses a novel "stop/fold" propulsion transition — the first of its kind to reach the X-plane fabrication stage. The concept:
- VTOL phase: Wingtip-mounted tilting rotors provide vertical lift and transition to forward flight.
- Cruise phase: Once airborne and accelerating, the rotors stop spinning and fold into the fuselage to eliminate drag. A rear-mounted turbofan engine provides jet-powered cruise.
- Unlike the V-22 Osprey, the rotors are not used for forward flight — only for vertical lift and transition. This is the critical architectural distinction.
Target performance specifications:
| Parameter | Specification |
|---|---|
| Cruise speed | 400–450 knots (740–833 km/h) |
| Altitude | 15,000–30,000 ft |
| All-up weight | 8,000–15,000 lb (3,600–6,800 kg) |
| Launch surface | Unprepared/austere — no runway required |
| Crewing | Unmanned demonstrator (manned/optionally-piloted at scale) |
These numbers are ambitious. 450 knots from a standing vertical start, without a runway, in a vehicle weighing up to 6,800 kg — this would be a class of aircraft that does not currently exist.
2. Why Bell, Not Aurora
Aurora Flight Sciences proposed a blended-wing-body with embedded fan-in-wing lift fans — aerodynamically novel, with projected speeds above 520 mph. Aurora completed wind tunnel testing during Phase 1B confirming ground effect performance. The National Interest published a piece questioning whether DARPA chose the wrong design.
The selection logic appears to favour technology readiness over aerodynamic novelty. Bell brings decades of tiltrotor heritage — the V-22 Osprey, the V-280 Valor (which won the Army's FLRAA competition) — and demonstrated stop/fold risk reduction during Phase 1B, including folding rotor demonstrations, integrated propulsion testing, and flight control technology demonstrations at Holloman Air Force Base, plus wind tunnel testing at NIAR/Wichita State University.
For a programme co-sponsored by SOCOM with operational urgency driven by Indo-Pacific scenarios, selecting the contractor with a deeper integration track record over the more novel but less proven concept is a defensible choice. Whether it is the right one will depend on whether the stop/fold transition — an unproven mechanism at these speeds and weights — performs as modelled.
3. Programme Timeline and Budget
| Phase | Period | Status |
|---|---|---|
| Phase 1A — conceptual design (4 contractors) | Nov 2023 – mid 2024 | Complete |
| Phase 1B — preliminary design (Aurora, Bell) | May 2024 – mid 2025 | Complete |
| Phase 2 — detailed design, fabrication, ground test (Bell) | Jul 2025 – 2027 | In progress |
| Phase 3 — flight test | 2028 | Planned |
Budget trajectory shows steady ramp-up:
| Fiscal Year | SPRINT Budget |
|---|---|
| FY2024 | US$13 million |
| FY2025 | US$42 million |
| FY2026 (requested) | US$55.2 million |
The FY2026 request represents a roughly 30% increase over FY2025 — consistent with a programme moving from design into fabrication. Bell's specific Phase 2 contract value has not been publicly disclosed.
4. The SOCOM Requirement
SPRINT is not a DARPA curiosity project — it is jointly run with US Special Operations Command, which has pursued high-speed VTOL as a formal requirement since at least 2021. The driving operational scenario is the Indo-Pacific: long distances, dispersed basing, contested airspace, and few runways.
SOCOM officials have stated HSVTOL would "close those distances in much more operationally relevant timelines." The concept of operations: SOF insertion and extraction at near-jet speeds from unprepared surfaces — ships, clearings, forward operating bases without improved runways — anywhere within a 315+ km radius.
The X-plane is a technology demonstrator, not a programme of record. No public information exists on what happens post-demonstration — whether this feeds directly into a SOCOM acquisition programme or remains a proof-of-concept for broader application. DARPA's stated goal is a demonstrator that validates concepts scalable to different-sized military aircraft.
5. Adjacent Programmes
DARPA ANCILLARY (AdvaNced airCraft Infrastructure-Less Launch And RecoverY) is a complementary programme at a much smaller scale — maximum 330 lb gross takeoff weight UAS with 12+ hour endurance, VTOL from ships and austere land sites. Six contractors (including AeroVironment, Northrop Grumman, and Sikorsky) entered flight testing in summer 2025. DARPA intends transition to military services by end of 2025. This is a different class entirely from SPRINT's 8,000–15,000 lb platform, but the infrastructure-less launch concept is shared.
Bell's broader HSVTOL portfolio extends beyond SPRINT. Bell has been developing the stop/fold concept independently and pitching it to multiple services. The SPRINT X-plane win validates their architecture as the leading high-speed VTOL approach — a commercial advantage that extends well beyond this single programme.
6. Assessment
SPRINT is on track and accelerating. The budget is ramping, the contractor is selected, and fabrication is under way. The 2028 flight test target is ambitious for a clean-sheet X-plane with a novel propulsion transition mechanism, but the programme shows no public signs of the schedule pressure afflicting other DARPA-adjacent efforts (notably HACM).
The significance is less about the demonstrator itself and more about what it validates. If Bell's stop/fold architecture works at the specified speeds and weights, it opens a design space for military VTOL that current tiltrotors (V-22, V-280) cannot reach. The V-22 cruises at roughly 240 knots; SPRINT targets nearly double that without a runway. That delta — if achieved — would reshape SOF aviation, carrier-based logistics, and potentially tactical airlift.
The open question is transition. DARPA demonstrates; services acquire. Whether SOCOM or another service takes SPRINT's output into a programme of record depends on the demonstration's success and on whether the stop/fold mechanism proves reliable enough for operational use, not just test flight. The 2028 flight campaign will determine which path this takes.
Sources and Further Reading
- Breaking Defense — DARPA picks Bell for Phase 2, July 2025
- Bell Newsroom — Phase 2 announcement
- Air and Space Forces Magazine — Bell selection
- Breaking Defense — Phase 1B awards, May 2024
- National Interest — "Did DARPA choose the wrong design?"
- Flight Global — SOCOM HSVTOL requirement
- DARPA — SPRINT programme page
- Forecast International — Bell's win signals tiltrotor future
