George had the story written long before A New Hope was made.

Wikipedia has a pretty good treatment of the history (of course):

I doesn't seem to be completely so:

In 1971, Universal Studios agreed to make American Graffiti and Star Wars in a two-picture contract, although Star Wars was later rejected in its early concept stages. American Graffiti was completed in 1973 and, a few months later, Lucas wrote a short summary called "The Journal of the Whills", which told the tale of the training of apprentice C.J. Thorpe as a "Jedi-Bendu" space commando by the legendary Mace Windy.[22] Frustrated that his story was too difficult to understand, Lucas then wrote a 13-page treatment called The Star Wars, which had thematic parallels with Akira Kurosawa's The Hidden Fortress.[23] By 1974, he had expanded the treatment into a rough draft screenplay, adding elements such as the Sith, the Death Star, and a protagonist named Annikin Starkiller. For the second draft, Lucas made heavy simplifications, and introduced the young hero on a farm as Luke Starkiller. Annikin became Luke's father, a wise Jedi knight. "The Force" was also introduced as a supernatural power. The next draft removed the father character and replaced him with a substitute named Ben Kenobi, and in 1976 a fourth draft had been prepared for principal photography. The film was titled Adventures of Luke Starkiller, as taken from the Journal of the Whills, Saga I: The Star Wars. During production, Lucas changed Luke's name to Skywalker and altered the title to simply The Star Wars and finally Star Wars.[24]

At that point, Lucas was not expecting the film to become part of a series. The fourth draft of the script underwent subtle changes that made it more satisfying as a self-contained film, ending with the destruction of the Empire itself by way of destroying the Death Star. However, Lucas had previously conceived of the film as the first in a series of adventures. Later, he realized the film would not in fact be the first in the sequence, but a film in the second trilogy in the saga. This is stated explicitly in George Lucas' preface to the 1994 reissue of Splinter of the Mind's Eye: It wasn't long after I began writing Star Wars that I realized the story was more than a single film could hold. As the saga of the Skywalkers and Jedi Knights unfolded, I began to see it as a tale that could take at least nine films to tell—three trilogies—and I realized, in making my way through the back story and after story, that I was really setting out to write the middle story. The second draft contained a teaser for a never-made sequel about "The Princess of Ondos," and by the time of the third draft some months later Lucas had negotiated a contract that gave him rights to make two sequels. Not long after, Lucas met with author Alan Dean Foster, and hired him to write these two sequels as novels.[25] The intention was that if Star Wars were successful, Lucas could adapt the novels into screenplays.[26] He had also by that point developed an elaborate backstory to aid his writing process.[27]