The Hitchhiker's Guide to the Galaxy Radio Series

Douglas Adams was asked to pitch a radio sitcom in February 1977. Originally to be called The Ends of the Earth, each episode would have ended with the planet Earth meeting its demise in a different way. While writing the first episode, Adams realized that he needed a character who knew what was going to happen to Earth before the other characters. He decided to make this character an alien and, remembering an idea he supposedly had while lying drunk in a field in Innsbruck, Austria in 1971, decided that this character would be a "roving reporter" for The Hitchhiker's Guide to the Galaxy. Later recollections by his friends at the time indicate that Adams first spoke openly of the idea of "hitch-hiking around the galaxy" while on holiday in Greece in 1973.

As the first radio episode's writing progressed, the Guide became the central focus of his story, and he decided to base the whole series around it, with the initial destruction of Earth being the only hold-over from the "Ends of the Earth" proposal. In Adams' February 1977 outline, the character of Arthur Dent was initially called "Aleric B", the joke being that the audience initially assume the character is also an alien rather than a human.

So if this is to be believed, the radio series predated the book. And it is also hard to believe that Adams really came up with this “story” randomly, since the events did actually happen.