The highly successful Mills Brothers inspired a large number of African-American singing ensembles in the decades of the Great Depression and the Second World War. Using only their voices and sometimes sparse instrumentation (guitars or tipples, which were a 10-stringed kind of ukulele), these groups combined jazz, pop, and gospel to produce recordings and styles that anticipated the rise of R & B, rock ‘n roll, and doo-wop in the 1950s.
We'll hear the Spirits of Rhythm (featuring legendary hipster and scatter Leo Watson), the Golden Gate Quartet (who helped pioneer the "jubilee" gospel sound), the Four Vagabonds (an important transitional group between the jazz-jive vocal groups of the late 1930s and the black R and B groups of the 1950s), Cats and the Fiddle, the Ravens (their 1947 recording of "Ol' Man River," which included a bass vocal lead by Jimmy Ricks that served as a harbinger of the doo-wop movement to come), and more.
There's more on black vocal harmony groups here, and you can also watch the Golden Gate Quartet doing "Hit the Road to Dreamland" with Mary Martin and Dick Powell (from the 1942 movie Star Spangled Rhythm):