Today in History - 1937

The longest-running detective show on radio debuted. Mr. Keen, Tracer of Lost Persons lasted until 1955 with 1690 nationwide broadcasts. Three different actors played the title role, Bennett Kilpack was Mr. Keen the longest, and Arthur Hughes saw the final show. Phil Clark also played the part.

And here's a link to a broadcast originally aired 9/15/1949. With more episodes here. [Real Audio required.]