From SG Glossary's "Space Opera" entry:
The Star Wars movies themselves owe much to Edmond Hamilton's 1949 novel The Star Kings and its sequel Return to the Stars.
However, having read Star Kings, I'm at a bit of a loss as to why that would be true. Leaving aside the fact that both works have a "princess" and a war in space, I'm unable to come up with SPECIFIC things linking the two. So, what I'm looking for is either:
Specific acknowledgement by Lucas or whoever that Star Wars was in some ways influenced by The Star Kings.
Or a good example of some Star Wars detail/idea that seems to be borrowed from/heavily influenced by The Star Kings (and is not widespread enough that it could equally have come from a bunch of other space operas / other sources). E.g. having a princess is kind of non-unique.
Or, a statement from Lucas to the effect that the influence was more that The Star Kings helped promote/popularize idea of "space opera".