The idea that Superman may be hiding in plain sight as a regular guy is voiced for the first time in Action Comics #9, January 1939. I like how Jerry Siegel seems to be building this world that Superman inhabits month-by-month, even if the story ideas themselves are sometimes a bit ludicrous. I also love the joy that Superman seems to be having in pulling the wool over everyone’s eyes (especially Lois’).
By this time, I think it’s fair to say that DC realizes that Superman is a hit, since every Action comic book cover either depicts or mentions Superman in some way going forward. Furthermore, the Superman newspaper comic strip began distributing in January 1939 with its first mentions of Superman’s home world (Krypton), father (Jor-L) and mother (Lora):
It is in the newspaper strip that the Superman mythos begins to take further shape, naming his home world, describing the death of the planet and the efforts of his scientist father.