The National Defense Act of 1920 was sponsored by United States Representative Julius Kahn, Republican of California. This legislation updated the National Defense Act of 1916 to reorganize the United States Army and decentralize the procurement and acquisitions process for equipment, weapons, supplies and vehicles. It was passed by Congress on June 4, 1920.

The National Defense Act of 1916, Pub. L.Tooltip Public Law  64–85, 39 Stat. 166, enacted June 3, 1916, was a United States federal law that updated the Militia Act of 1903, which related to the organization of the military, particularly the National Guard. The principal change of the act was to supersede provisions as to exemptions. The 1916 act included an expansion of the Army and the National Guard, the creation of an Officers' and an Enlisted Reserve Corps, and the creation of a Reserve Officers' Training Corps. The President was also given expanded authority to federalize the National Guard, with changes to the duration and the circumstances under which he could call it up. The Army began the creation of an Aviation arm, and the federal government took steps to ensure the immediate availability of wartime weapons and equipment by contracting in advance for production of gunpowder and other material.