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Bell X-1 in flight.

Source: http://afftc.edwards.af.mil/history/docs_html/aircraft/x-1_test_program.html アーカイブされたコピー at the Wayback Machine (photo link in first paragraph). Relevant description:

During World War II, fighter pilots encountered a new and terrifying phenomenon. Rolling over into steep dives, they accelerated to speeds of 500 mph and into the unknown region of transonic flight (0.7-1.3 Mach) where the effects of compressibility--loss of control and structurally devastating aerodynamic loads--began to take over with often deadly consequences. By war's end, new turbojet engines were under development and they promised even higher speeds--speeds passing through the transonic and even, perhaps, into the supersonic region. So little was known about transonic aerodynamics, however, that many aerodynamicists theorized that drag would reach infinity as an airplane approached the speed of sound. The possible existence of a "sound barrier" was only one of a host of unknowns constituting a very real barrier to flight progress. Aircraft designers could not proceed without valid data and the wind tunnels of the day, which "choked" as the airflow around models reached transonic velocities, provided few answers. Thus an experimental research airplane--the rocket-powered and bullet-shaped Bell X-1 (photo)--was designed and built to acquire the necessary data...and to determine whether or not a piloted aircraft could actually penetrate the "sonic wall." The X-1 was the first in a series of "X"--or experimental--aircraft that were designed to answer fundamental questions, to probe the most challenging unknowns of flight and solve their mysteries. The program was also the Air Force's first foray into experimental flight research and the first collaborative effort in what would become an extraordinarily productive partnership between it and the National Advisory Committee for Aeronautics (NACA). The first NACA contingent arrived at Muroc Army Air Field (later Edwards) in September of 1946 and the NACA and its successor, NASA, have been conducting fundamental flight research there ever since.