Gunsmoke was on radio for 9 seasons, from 1952 to 19612. Six of those seasons coincided with the television series.

There were 413 radio stories, broadcast 480 times.

From an e-mail from Mrs. Howard Culver (Howie, the Dodge House clerk from the TV series)

Yes, Howard was one of several who did auditions for the radio Gunsmoke, and he was chosen from the recordings with unnamed applicants to play Marshall Dillon. He cut a record (Matt Dillon was known as Mark Dillon in it), but since Howard was playing the lead in "Straight Arrow" on Mutual Network, his contract with Mutual would not let him play the lead in the Gunsmoke show. Thence it was given to William Conrad.