They might have; I encountered the first book in 1980 and I always thought that was the beginning of the franchise. But I'm often wrong.
Yes, the radio show came before the books. One of my prized possessions is a 2nd printing HHGTTG by Chaucer Press, UK in 1979, with all the words like 'color' spelled funny. It says 'based on the famous radio series' right on the cover and in the frontice piece says the radio show ran from August '78 to December '78.