I didn’t either, but it happened on the radio in 1975. The Fantastic Four radio show was the brainchild of a couple of New York DJs (Sean Kleefeld interviewed one of them in 1999) who pitched the idea to Marvel. The company went for it, and Stan Lee even provided the narration. The scripts were based entirely on actual Lee/Kirby comics and, yes, a 25-year-old Bill Murray played Johnny Storm.
The Kleefeld interview has all the details, including why the show only lasted for 13 episodes. And if you’re interested in hearing an episode for yourself … well, thank Galactus for YouTube.