Bluegrass music was born when country music met jazz. In 1945, when Bill Monroe's Bluegrass Boys were fully formed, jazz music was in its heyday. Sometimes we like to think that bluegrass music is its own entity, raised from the dirt in the hills and hollars of Kentucky and Tennessee. It's just not true. It's the blending of styles that created bluegrass music. Old-time country, square dance fiddle tunes, blues, gospel, ragtime, and yes, jazz.
Just look at these early Bill Monroe titles:
Honky Tonk Swing
These are all reflective of what was on the radio and records in the 1930's. Bluegrass owes much to "hillbilly" musicians being heavily influenced by jazz. Listen to the walking bass lines in "Tennessee Blues," the polyphony of sounds in "Heavy Traffic Ahead," and the joy of adventure in Bill's playing on "Little Cabin Home on the Hill." It's absurd to think that these country boys from the south somehow never heard Louis Armstrong or Benny Goodman.
I would go so far to say, as that Earl Scrugg's rolling banjo style would not have developed without the influence of Benny Goodman, who was on NBC radio every Saturday night for awhile in the 1930's. Here's "Bugle Call Rag," first by Benny Goodman, then by "Flatt and Scruggs."
Link: Benny Goodman - Bugle Call Rag
Link: Flatt and Scruggs - Bugle Call Rag