The Douglas X-3 Stiletto was a 1950s United States experimental jet aircraft with a slender fuselage and a long tapered nose, 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 its purpose and could not even exceed Mach 1 in level flight. Though the research aircraft was a disappointment, Lockheed designers used data from the X-3 tests for the F-104 Starfighter which used a similar wing design in a successful Mach 2 fighter.
Source: Wikipedia, "Douglas X-3 Stiletto", available under the CC-BY-SA License.