Star Wars was a kids' movie. "A long time ago in a galaxy far, far away" -- it tells us right up front that it's a fairy tale. It was Lucas's homage to all the stuff he loved as a kid, like Flash Gordon serials, samurai movies, WWII movies, Laurel and Hardy, etc. So it can't really be called the first of its kind when its entire reason for existence was to pay homage to its many predecessors. Unless you mean that it pioneered the mainstreaming of sci-fi and space opera into popular acceptance, but I think Star Trek has it beat there by over a decade. (And it was paying homage to Star Trek too, as is evident from the title.) Lucas has said that he cited Star Trek's success in syndication to convince studio execs that his space-opera movie could be profitable.