Around 1975 or 1976, he wrote down the character histories that exist mentally in every writers head. This resulted in a note collection of about "8 or 9 pages", in his words, which contained character sketches and background details, such as the formation of the empire and fall of the republic, the nature of the jedi, the broad history of people like obi wan and palpatine. Some of this found its way into the prologue of the novelisation in 1976, which, if you read, gives a pretty approximate parallel to the political-based plot of the PT.

So, in a sense, yes he sorta did, but in a larger sense, not really. He didn't plan it to be a series or ever shown on film, and it was vague and nothing special, the same background development that stems from any reasonably-complex story, and many of the pivotal prequel details (ie Darth Sidious, Anakin becoming Vader, Yoda, etc.) were absent and not added until ESB and ROTJ era.