While vigilantes and rogue policemen have existed for millennia, it wasn't until the Old West era in the western America that the first costumed crime-fighters appeared. The costumed superhero El Diablo hid his identity while fighting alongside other vigilantes, such as Bat Lash and Sheriff Ohiyesa Smith.
By the 1940s, the idea of costumed superheroes was still far-fetched in the minds of American soldiers, although at least one costumed vigilante helped out during World War II: Spy Smasher.
Superheroes in fiction
In popular fiction around the 1950s, the Justice Guild of America comics were published featuring a superhero team fighting against colorful villains. These concepts were based on an alternate Earth that tapped into the minds of comic book creators.
In the 1960s, an accident at the Hoover Dam gave a young Morris Grant power over electricity. Becoming the first metahuman hero, Grant took on the identity of Soul Power and was the first crime-fighter in Dakota. He would take on the sidekick Sparky and gain a powerful archenemy: Professor Menace.
A couple decades later, billionaire Bruce Wayne adopted a costumed identity to strike fear in the hearts of criminals after he discovered that infiltration did not work.
At some point, a Marine named John Stewart was recruited from Earth into the Green Lantern Corps becoming the Green Lantern. He spent much of his early career off-world and wasn't known until after the recruitment of another Green Lantern.
The first extraterrestrial superhero, and arguably best known by the public, was Superman, named by Lois Lane after appearing in Metropolis. He was able to use his Kryptonian powers to aid Metropolis and the world during a time when technology and alien contact had nearly caused massive disasters several times over.
Explosion of superheroes
In Gotham City, Batman had taken on several partners in his crime-fighting duties that were superheroes in their own right. He also helped both Jason Blood (a well-known occultist) and the Creeper become costumed crime-fighters.
Following the creation of the Justice League (see below), the amount of costumed superheroes seemed to increase exponentially.
Although superheroes often teamed up for various missions, it wasn't until after the Imperium invasion that a permanent crime-fighting force (discounting the Green Lantern Corps) was formed amongst superheroes. The Justice League, proposed by Superman, operated out of the Watchtower and took on forces that were too much for one hero.
Teams were formed in the wake of the Justice League, including the Titans and the Hoop Squad, and the Justice League itself reformed after the Thanagarian invasion to include most active crime-fighters (which had expanded to over sixty by that time).