Their designations reflected their use: fighters—also called pursuit planes—were assigned letters F or P: for example, the Chance Vought F4U Corsair or the North American P-51 Mustang. The letter C identified a cargo plane like the Douglas C-47 Skytrain, built to transport military goods and troops and, eventually, commercial passengers. B was for bomber, like the mammoth and perfectly named B-29 Superfortress. And X identified an experimental plane still under development, designed for the purpose of research and testing. Planes lost their X designation—the B-29 was the direct descendant of the XB-29—once they went into production. The same evolutionary forces prevailed to replicate a particular model’s positive traits and breed out excess drag and instability. The P-51A Mustang was a good plane; the P-51B and P-51C were great planes. After several rounds of refinement in the Langley wind tunnels, the Mustang achieved its apotheosis with the P-51D.