B-movies came into existence during the Great Depression. Money was scarce, so movie studios, who owned the theaters at the time, would show a second, cheaply-made second film along with the main, A-movie feature. These films were usually action movies or westerns and ran about sixty minutes in length. Some B-movies were produced by the major studios, although there were studios such as Monogram and Republic who only made B-movies.
This major studio monopoly over film production and distribution was made possible by President Roosevelt's National Industrial Recovery Act, which permitted the situation for the sake of promoting economic recovery. A late 1940s Supreme Court decision put a stop to this. Independent studios then became possible, and the additional competition raised production costs to the point that B-movies were no longer economical. Double bills ceased to exist, and B-movies as such were no longer produced.
The term "B-movie" continues to be applied to movies that exhibit the sensational subject matter or camp aesthetic that had become associated with these films. B-movies had always existed as the underbelly of American cinema, and so were permitted to contain riskier content than their mainstream cousins. This allowed them to emerge as an alternative, maverick genre. Film noir and horror and science fiction of the 1950s and early 1960s are genres that existed almost entirely under the B-movie umbrella.