Blake Edwards' hard boiled private eye, Richard Diamond, began on radio in 1949, and spent 4 years on three different networks before ending.
Actor-singer Dick Powell played Diamond, a former NYPD detective who left the force to move into the PI business. Every episode ended with Diamond serenading his girlfriend, Helen (Virginia Gregg), with a musical number, owing to Powell's previous run of movie musicals. The musical interludes were discarded when the series transitioned to television in 1957, and David Janssen was cast as Diamond.
To tell you the truth, having seen the TV show, I'm a little more comfortable with that than the radio show. But judge for yourselves while you listen to the series opener, courtesy of Internet Archive.
Powell retained the rights to the series, as his Four Star Productions produced the TV version.