Yours Truly, Johnny Dollar
Yours Truly, Johnny Dollar was a radio drama about the "fabulous freelance insurance investigator with the action-packed expense account". It aired from February, 1949 to September 30, 1962 on CBS, giving it an amazing 14-year run.
Charles Russell was the first to star as Johnny Dollar, followed by a succession of actors including Edmund O'Brien, John Lund, Bob Bailey, Bob Readick, and Mandel Kramer. Each new star gave Johnny a slightly different persona.
You can find more episodes in the Ear Benders section.
Here is a sample of the online episodes currently available at The Old Time Radio Fan: