The variety of music that was introduced to Americans during the Depression (1929–41) continued to evolve during the 1940s. Blues became electrified and transformed into rhythm and blues (R&B), boogie-woogie, and what became known as "jump blues," laying the foundations for the emergence of rock and roll in the coming decades. Blues music was now easily and cheaply recorded on new magnetic tape recorders. Another new musical style called "bebop" developed in jazz dance clubs in Manhattan and Chicago. Bebop offered talented musicians, such as trumpet players Dizzy Gillespie (1917–1993) and young Miles Davis (1926–1991) and piano player Thelonious Monk (1917–1982) a chance to shine. There was room in bebop for improvisation (spontaneous music-making) in the songs.
As part of the New Deal program to give people work, government researchers paid by the New Deal sought and recorded the folk music of America. The folk and hillbilly music was also changing, though. Like blues, country music incorporated elements of other styles, especially swing, and made use of electric and steel guitars, which moved it closer to what would become rock and roll. Country music was especially popular with the large populations of rural people who had moved to cities. But servicemen also loved the music. The Special Services Division of the European Theater of Operations organized a tour of country bands to the troops fighting in Europe. The Grand Ole Opry radio program was one of the most listened-to radio shows during the decade. Roy Acuff (1903–1992) was one of the most popular country singers. Like jazz, country music had an offshoot that valued improvisation. Bluegrass developed during the 1940s and highlighted banjo and guitar players' skills and featured songs of the hard life: drunkenness, joblessness, marital troubles, and regrets.
At the beginning of the decade, swing-jazz orchestras played in dance halls, on the radio, on the soundtracks to movies, and on single and long-playing records that were big sellers. These big bands often featured a popular singer, such as Billie Holiday (1915–1959), Bing Crosby (1904–1977), and Frank Sinatra (1915–1998), who crooned (sang in a smooth voice just right for amplification through a microphone) to audiences. After World War II (1939–45), the dynamic brass sounds of the big bands were replaced by more string instruments to develop a softer pop music.