The economic depression that the world experienced in the 1920s made the military spending during peacetime look excessive.
World War I drove aviation and pilots for improvement in flying skills and technology both for civil and military. This gave the military room to develop technology for war without being noticed.
The spitfire fighter was developed from a civilian racing plane, seeing its potential for war. In 1938 for instance a Seversky pursuit plane manned by a civilian won the Bendix Race. The military saw its abilities and immediately ordered same type of planes eventually developed into Republic P-47 Thunderbolt the most preferred plane for war. Acquiring technology from civilian gave the military operational and logistical experience.
Middle of 1920s, Germans started rearmament. Lufthansa, their leading airline, was nationalized to train pilots and set the aviation industry in motion. Transformation of Luftwaffe was the highest priority since the Nazi party came to power in 1930.
Readl also: The First Practical Airplane