The question ‘who was the first DC superhero’ is ambiguous. On the one hand, it could refer to the first superhero to be published by DC Comics - this could arguably be Superman. On the other hand, it could refer to the first superhero within DC continuity - this wouldn’t be Superman. But who was really the first DC Comics superhero and who was the first hero in continuity?
The first thing to acknowledge when discussing the DC Universe is that it has a deep and vast history that stretches far beyond the current events. Consequently, in current canon, even if Superman was the ‘first’ superhero to appear overtly to the modern world, there have been scores of other heroes that would have been operating in the past. Heroes such as Hawkgirl and Hawkman, Etrigan, and Dr. Mist (using pre-crisis continuity) operated far before Superman and, taking their characterizations into account, would have probably done so regardless of any considered continuity.
While the examples presented above are certainly interesting, there is one particular character whose origins extend to the creation of the universe itself. The Spectre is a character who has been operating in the fringes of mainstream DC continuity for some time. While the Spectre has played his part in some of the biggest DC stories of all time, the character also has very little recognition in terms of mainstream culture.
For those who are unaware of the Spectre, the character is a being that has been around since the dawn of time. He is an embodiment of God’s wrath, created to cleanse the world of evil and inequity. Due to this character’s Old Testament-esque description, the Spectre is generally described as a brutal character who is closer to that of an anti-hero than a true hero who would comfortably fit in with the Justice League.
While Superman isn’t the first superhero to exist in DC canon, he’s still the first DC superhero to be published, right? This largely depends upon one’s definition of a superhero. If a superhero is a character who fights crime in tight spandex, then he certainly would be. However, if your definition of a superhero is slightly vaguer, then this may not be the case. Famously, Superman debuted in Action Comics #1 in 1938, becoming an instant success in the process. Although, Action Comics wasn’t the first comic book line produced by the publisher.
In 1935, National Allied Comics (which existed before DC Comics) had another comic series that was quite popular: New Fun Comics. In issue six of this series, a character named Doctor Occult appeared. This character was a private investigator who practices magic, giving him the power to astral project, cast illusions, and the ability to use telekinesis. While the character may not be too famous today, Doctor Occult had an important role in the success of superheroes, Superman and DC Comics. Doctor Occult was created by Jerry Siegel and Joe Shuster, who created Superman. It could be argued that, without Doctor Occult, Superman and DC Comics as we know it may not exist in the same form today. Superman might be the first big superhero, however, he's not the first superhero ever - at least under further context.