The Justice Society of America debuts in All-Star Comics #3 (November 1940). In its first incarnation it consists of a whopping nine characters across Detective Comics Inc and All-American Comics (still separate companies at this point): The Flash, the Atom, Green Lantern, Hawkman, the Spectre, Doctor Fate, Sandman and Hourman, with Johnny Thunder thrown in as comic relief. Batman and Superman are only given mentions, since they have well-established careers at this point and don’t need the readership boost.
It is the first example of a superhero team, bringing together many characters across different books together in a single universe continuity. This issue also may be the first time an entire comic book is dedicated to a single story. In reality, the book has a wrapping story that exists as a framing device for individual character stories, but it’s still a pretty momentous milestone that would shape comic books for decades to come.