It's no secret that film composers often borrow a great deal of musical material from other sources. In fact, directors usually provide their composers with "temp tracks", place-holder music that they want the film score to sound like. The composer then takes those suggestions and writes in the style of those temp tracks, but in a way custom-built to fit the film. Given the similarities between the John Williams' main theme to Star Wars (1977) and Erich Korngold's main theme to King's Row (1942), it seems to be a safe bet that George Lucas had King's Row on his temp track for Star Wars.
While it is easiest to hear the similarities, here is a transcription, as well, for visual comparison.
Both tunes are in B-flat major and share a heroic, brassy quality. Seven of the first eight tones are completely identical (indicated by the solid red lines), with the second note of each being comparable but not identical (indicated by the dotted red line). Further notice that the two circled triplets are retrogrades of each other (Star Wars goes Eb-D-C; King's Row goes C-D-Eb), and the two passages with the rectangles are similar in that they both descend from Eb to C and use a triplet.
In this case, John Williams has borrowed the first several notes from a pre-existing theme, then spun the rest of that theme from there. This is a pattern that will appear quite frequently throughout the Star Wars films.
The Music of Star Wars
These posts will help focus and develop my analyses of John Williams' film scores.