The short answer is that no, the prequels were never supposed to be made into films, and Lucas has actually been very upfront about this. In fact, the first drafts of ESB were titled as "Episode II"! The prequel trilogy came into existance in 1978 (possibly even 1979) once Lucas merged the characters of Darth Vader and Father Skywalker aka Anakin Skywalker (they were originally seperate people up until April of 1978).