The major thrust of aerodynamics in the age of the advanced propeller-driven airplane can be summarized in a word: streamlining. Research in aerodynamics in that age, generally covering the period from about 1930 to 1945, was driven by two practical concerns: (1) the need to reduce the drag on aircraft so they could fly faster and more efficiently, and (2) the problems associated with fluid compressibility as new, high performance propeller-driven aircraft began to reach speeds approaching the speed of sound. That age was characterized by intensive efforts to better understand the mechanisms of drag production, to find ways to better predict drag, to meticulously refine a configuration in order to reduce drag, and to begin to sneak up on the speed of sound without disastrous consequences. The outcome of all that effort was the trend toward more streamlining of aerodynamic bodies.

It was also during that age that the fundamental innovative ideas in aerodynamics that had been articulated at the turn of the century finally began to yield great rewards. Prandtl's boundary-layer theory, first presented in 1904, spread throughout the world of aerodynamics in the late 1920s, allowing airplane designers to make the first intelligent (and sometimes reasonably accurate) predictions of skin-friction drag. The boundary layer concept also helped to explain how flow could separate from a surface; such flow separation is the cause of form drag (pressure drag due to flow separation).