The Douglas X-3 Stiletto, intended for advanced Mach 2 turbojet propulsion testing, was a 1950s United States experimental jet aircraft with a slender fuselage and a long tapered nose and was the sleekest of the early experimental aircraft.
Manufactured by the Douglas Aircraft Company its primary mission was to investigate the design features of an aircraft suitable for sustained supersonic speeds, which included the first use of titanium in major airframe components. It was, however, seriously underpowered for this purpose and could not even exceed Mach 1 in level flight.
Construction of a pair of X-3s was approved on 30 June 1949. During development it was found that the X-3's planned Westinghouse J46 engines were unable to meet the thrust, size and weight requirements. Lower-thrust Westinghouse J34 turbojets were substituted, producing only 4,900 lbf (21.8 kN) of thrust with afterburner rather than the planned 7,000 lbf (31.3 kN).
The first aircraft was completed and delivered to Edwards Air Force Base, California, on 11 September 1952. Due to engine and airframe problems, the partially completed second aircraft was cancelled and its components were used for spare parts.
The official first flight, lasting about 20 minutes, was made on 20 October 1952. Test flights continued until the end of December 1953. These showed that the X-3 was severely underpowered and difficult to control. During testing the X-3's fastest flight, made on 28 July 1953, reached Mach 1.208 in a 30° dive. Its take-off speed was an unusually high 260 kn (482 km/h).
With the completion of the contractor test program in December 1953, the X-3 was delivered to the United States Air Force. The initial flights looked at longitudinal stability and control, wing and tail loads. By late October, the research program was expanded to include lateral and directional stability tests. These tests would lead to the X-3's most significant flight and the near-loss of the aircraft.
On 27 October 1954 NACA test pilot Joseph A. Walker made an abrupt left roll at Mach 0.92 and an altitude of 30,000 ft (9,144 m). The X-3 rolled as expected, but also pitched up 20° and yawed 16°. The aircraft gyrated for five seconds before Walker was able to get it back under control. He then set up for the next test point putting the X-3 into a dive, accelerating to Mach 1.154 at 32,356 ft (9,862 m), where he made an abrupt left roll. The aircraft pitched down and recorded an acceleration of -6.7 g (-66 m/s²), then pitched upwards to +7 g (69 m/s²). Walker managed to bring the X-3 under control and successfully landed.
Post-flight examination showed that the fuselage had been subjected to its maximum load limit. Had the acceleration been higher, the X-3 could have broken up. The aircraft was grounded for nearly a year after the flight, and never again explored its roll stability and control boundaries. Walker made another ten flights between 20 September 1955 and 23 May 1956.
The X-3 was subsequently retired to the U.S. Air Force Museum. However, the X-3 left a legacy when Lockheed designers used data from the X-3 tests for the Lockheed F-104 Starfighter which used a similar trapezoidal wing design in a successful Mach 2 fighter.