Which bandleader did Frank Sinatra sing with before joining Tommy Dorsey?

Before joining Tommy Dorsey, Frank Sinatra sang with bandleader and trumpeter Harry James. Sinatra joined Harry James and His Orchestra in 1939, an important early step in his professional career.

During his short time with the James orchestra, Sinatra made recordings including “From the Bottom of My Heart” and “All or Nothing at All.” The latter became especially important in retrospect: although it was not an immediate hit, later reissues helped introduce listeners to the young singer's developing style.

In early 1940, Tommy Dorsey hired Sinatra away from James. Dorsey's orchestra gave Sinatra national exposure and a powerful musical education, especially through Dorsey's smooth trombone phrasing. Sinatra's work with the band and the Pied Pipers produced major hits such as “I'll Never Smile Again,” launching him toward solo stardom.

The common mix-up is to name Tommy Dorsey himself, since Dorsey was the bandleader who made Sinatra famous. But the question asks who Sinatra sang with immediately before Dorsey: that earlier orchestra was led by Harry James.