World War II was . . . responsible for considerable changes in the U.S. federal income tax. Not only were rates increased, but the base was extended to cover most of the working population. Even as late as 1939, only 6 percent of U.S. citizens had to file an income tax return; by 1945 this had increased to over 70 percent . . . By 1945 the major features of the current federal tax system were in place.
Susan B. Hansen, The Politics of Taxation: Revenue Without Representation, New York: NY, Praeger (1983) p. 86.