The famous Bell X-1 was the first aircraft to break the sound barrier in level flight. Its development started in 1945, and it was inspired by the shape of a Browning .50-cal machine gun round. The first supersonic test flight occurred on 14 October 1947. A derivative ordered by the US Air Force (USAF) in 1948 was the Bell X-1A, which was an improved design featuring greater fuel capacity and hence a loner engine-burn time. The X-1A looked slightly different with its bubble canopy and longer fuselage, although it was powered by the same Reaction Motors XLR-11 rocket engine. It was intended for research of aerodynamic properties at speeds surpassing Mach 2. The Bell X-1A (with serial number 48-1384) first exceeded Mach 2 in November 1953, but the following month test pilot Captain “Chuck” Yeager achieved a speed record of Mach 2.44. Unfortunately, the experimental plane was later lost in an explosion in August 1955.