Marvel's first entry debuted in 1966. The Marvel Super Heroes aired five times a week with a different hero taking the stage each night (each with his own theme song). The series pretty much consisted of straight adaptations of comic book series onto film. In fact, the animation costs were kept to a minimum by photocopying art from the comic pages and just animating the mouths.
Monday night belonged to Captain America (along with occasional Avengers team-ups)
Tuesday was Hulk night
Iron Man took the stage on Wednesday
Thor was all over Thursday
Prince Namor the Sub-Mariner finished the week on Friday
The X-men actually made their onscreen debut in the Prince Namor series, replacing the source story's Fantastic Four due to rights issues.
In all, 195 7-minute segments were produced for 65 episodes.