All three built were lost in fatal crashes. Similar to the D.H. 108, the twin-jet powered 1948-vintage Northrop X-4 was one of the series of postwar X-planes experimental aircraft developed in the United States after World War II to fly in research programs exploring the challenges of high-speed transonic flight and beyond. It had aerodynamic problems similar to those of the D.H.108, but both X-4 examples built survived their flight test programs without serious incidents through some 80 total research flights from 1950-1953, only reaching top speeds of 640 mph (1,035 km/h). The French Mirage series of supersonic jet fighters were an example of the tailless delta configuration, and became one of the most widely produced of all Western jet aircraft. By contrast the Soviet Union's equivalent widely produced delta-winged fighter, the Mikoyan-Gurevich MiG-21, does have a tail stabiliser. In the 1950s, the Convair F2Y Sea Dart prototype became the only seaplane ever to exceed the speed of sound. Convair built several other successful tailless delta types. The Anglo-French Concorde Supersonic transport and its Soviet counterpart the Tupolev Tu-144 were tailless supersonic jet airliners, with gracefully curved "ogival delta" wings. The grace and beauty of these aircraft in flight were often remarked upon. The American Lockheed SR-71 Blackbird reconnaissance aircraft was the fastest jet powered aircraft at the time it was retired, achieving speeds above Mach 3. The NASA Preliminary Research Aerodynamic Design To Lower Drag (PRANDTL-D) wing has been developed by Al Bowers at the NASA Armstrong Flight Research Center. Bowers was inspired by the work of Ludwig Prandtl and, like Dunne, by watching bird flight. As with the Dunne design, it has a wing twist sufficient to set the wing tips at a negative angle and create the same positive roll-yaw coupling. Bowers developed a quantitative analysis of the lifting characteristics, leading to his more general discovery of a bell-shaped lift distribution which minimises induced drag for the aircraft weight. He applied this distribution in the "Prandtl-D" series of designs. By the end of 2017, he had flown three such research models.