Answer by taking a quote from the following article:

Harry Lillis "Bing" Crosby Jr. (; May 3, 1903 - October 14, 1977) was an American singer and actor. Crosby's trademark warm bass-baritone voice made him one of the best-selling recording artists of all time, having sold over one billion analog records and tapes, as well as digital compact discs and downloads around the world.

Initial successes with Whiteman were followed by disaster when they reached New York and Whiteman considered letting them go. Bing may have been retained as Whiteman was already using him as a solo performer on record, but the prospects for Rinker were bleak. However, the addition of pianist and aspiring songwriter Harry Barris made the difference and "The Rhythm Boys" were born. The additional voice meant the boys could be heard more easily in the large New York theaters and eventually became a success. A year touring with Whiteman performing and recording with musicians Bix Beiderbecke, Jack Teagarden, Tommy Dorsey, Jimmy Dorsey, Eddie Lang and Hoagy Carmichael, provided valuable experience and began touring. Crosby then matured considerably as a performer and was in constant demand as a solo artist.  Crosby soon became the star attraction of the Rhythm Boys, and in 1928 he had his first number one hit with the Whiteman orchestra, a jazz-influenced rendition of "Ol' Man River". In 1929, the Rhythm Boys appeared in the film The King of Jazz with Whiteman but Bing's growing dissatisfaction with Whiteman led to the Rhythm Boys leaving his organization. They joined the Gus Arnheim Orchestra performing nightly in The Coconut Grove of the Ambassador Hotel. Singing with the Arnheim Orchestra, Bing's solos began to steal the show, while the Rhythm Boys act gradually became redundant. Harry Barris wrote several of Crosby's subsequent hits including "At Your Command", "I Surrender Dear", and "Wrap Your Troubles In Dreams". In the early months of 1931, a solo recording contract came Bing's way, Mack Sennett signed him to make film shorts and a break with the Rhythm Boys became almost inevitable. Bing had married Dixie Lee in September 1930 and after a threatened divorce in March 1931, he started to apply himself seriously to his career. His gramophone records in 1931 broke new ground as his powerful and emotional singing started to change the face of popular music forever.  Their low salaries at the Cocoanut Grove of the Ambassador Hotel had led the Rhythm Boys to walk out, causing union problems for Bing. Bing's brother, Everett, interested Bill Paley of CBS in his brother and Paley beckoned Bing to come to New York. A settlement was reached with the Ambassador Hotel and Bing made his first solo national radio broadcast in September 1931 and then went on to star at the New York Paramount Theatre.

who are the rhythm boys?
However, the addition of pianist and aspiring songwriter Harry Barris made the difference and "The Rhythm Boys" were born.