In 1963, Marvel really got hopping. For September of that year, they turned out not one, but two new superhero teams. The first, The Avengers, was made up of the heroes they had launched to that point, including Iron Man, Thor, Ant-Man, and the Hulk. The second group, therefore, had to be newly created. Rather than give them separate origins, Stan and Jack came up with a new concept for superheroes: They had all been born that way. They were mutants, members of "homo superior", as compared to homo sapiens.
Here's the cover:
The Flyboy is The Angel, originally Warren Worthington III. Although this issue does not reveal much more about him than that he can fly and wears a harness while in street clothes that hides his wings, the name is clearly intended to give us an image of a wealthy young preppie.
The big-footed gentleman on the trapeze is The Beast, real name Hank McCoy. He's something of a monkey-boy, capable of bouncing around at will and using his feet almost like hands. He's also a sesquipedalian, somebody who never uses a simple word when a six-syllable term will suffice.
The laser guy is Cyclops, introduced in this issue as "Slim" Summers although later stories use his given name of Scott. Although it is not apparent in the first issue, Summers is the leader of the team when away from their home base.
The gal in the background looking like she's riding a surfboard is Jean Grey, alias Marvel Girl. Her power is here described as "teleportation" although in fact it is more telekenesis as later stories will acknowledge. Jean is the source of some of the friction in the group, as everybody (except one) is attracted to her.
The lone holdout from the Jean Grey admiration society (at least in this first issue) is Iceman, shown throwing snowballs above. Bobby Drake is the youngest of the X-Men at only 16.
The villain is Magneto, certainly one of the most durable villains of the Marvel Silver Age. He too is a mutant, but an evil mutant determined to take over the Earth as its ruler.
Not shown is the regular leader of the X-Men, Professor Xavier. Although confined to a wheelchair, he possesses a superior mind, capable of telepathy at great distances.
The X-Men were destined to become one of the great teams in comic book history, although that may not have been obvious at first. In fact, they did not survive to the end of the Silver Age, being cancelled after issue #66, March 1970. But they would return in the mid-70s with different members and become extremely popular.