This
comic book was published in 1938 and featured the appearance of the very first costumed hero:
Lee Falk's
The Phantom. Up until now comic books were largely.. well, comical. Hence the name. The introduction of a costumed vigilante started a chain reaction.
Ace Comics #11 marks the end of the Pre-Golden Age period in comic book history. Soon after this, Action Comics #1 was published, in which Superman first appeared, thus introducing the Golden Age.
So if anyone ever tells you Superman was the first costumed hero, they're wrong. Phantom came first.