Let me quote my own dissertation here:
The second-draft screenplay, finished in January 1975 is titled The Adventures of the Starkiller, Episode One of the Star Wars
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On May 1st 1975 Lucas produced a new six page synopsis which he sent to 20th Century Fox to inform them about the development of the story. This new synopsis was basically a short summary of the second draft screenplay
In May 1975 Lucas decided to rename The Star Wars to Episode Four in the Adventures of Luke Starkiller. At this point however this decision probably wasn’t made because of the planned prequels; In the Unauthorized Star Wars Compendium the producer of Star Wars Garry Kurtz is quoted to have said in an interview for Starlog magazine:
"Certain kinds of movies just work better – or seem to work better – when you come in in the middle. I know that was the case with a lot of the low-budget films I made with Roger Corman. When you arrive in the middle, you miss all the tedious exposition and jump right into the action. We wanted that effect with Star Wars – the feeling that you had come in after the movie had started." (Edwards 13)
This idea was later dropped however, and the third draft of the script was titled Episode I again (Kaminski 104) and the final film was simply called Star Wars and the episode title was omitted completely. It was only after the huge success of the film and during the production of the first sequel that Lucas decided to go back to this concept and dub the sequel Episode V and then he retroactively renamed Star Wars to Star Wars Episode IV A New Hope in 1981. (Kaminski 230)