How did rhythm and blues develop?

The migration of African Americans to the urban industrial centers of Chicago, Detroit, New York, Los Angeles and elsewhere in the 1930s created a new market for jazz, blues, and related genres of music, often performed by full-time musicians, either working solo or in small groups. The precursors of rhythm and blues came from jazz and blues, which overlapped in the 1930s through musicians such as Leroy Carr, Cab Calloway, Count Basie, and T-Bone Walker. There was also increasing emphasis on the electric guitar as a lead instrument, as well as the piano and saxophone.