Recently I had the chance to read DC's SHOWCASE reprints of the earliest issues of CHALLENGERS OF THE UNKNOWN, which includes all of the issues on which Jack Kirby worked. COTU, as the heroic team's name is often abbreviated, is often considered by many Kirby fans to be evidence that Jack Kirby probably brought some work very like the Challengers to Marvel in the early 60s, and that this was eventually altered to become the company's flagship superhero title, 1961's THE FANTASTIC FOUR.
The proposition is certainly a fair one. All artists tend to recycle work that either interests them or their audience (in terms of selling well), so it's logical to assume that one heroic team might beget another.
In addition, for many fans the most salient similarity between the two hero-teams is that they bond after a crash landing. In the origin of the Challengers from 1957's SHOWCASE #6, the four heroes are unrelated adventure-seekers who happen to share a plane on the way to a mutual rendezvous. When all four men survive their plane's crash, they decide that they now live on "borrowed time" and to band together in order to perform feats of derring-do.
As most fans know, this has some resemblance to the origin of the Fantastic Four as told in the premiere issue of their magazine (though details were often changed in later retellings). The most salient difference is that all four of the heroes in FF #1 know one another quite well when they agree to Reed Richards' idea that they should steal the rocket he worked on from the government and use it to fly to the moon. In addition, whereas the crash in SHOWCASE #6 happens by sheer chance, the cause of the FF's rocket-crash stems from a peril that Reed and his allies foresee: a barrier of cosmic radiation on the outskirts of Earth's atmosphere. All four would-be astronauts find themselves transformed into cosmically-powered beings, with the result that, even though one of them has become a rock-skinned monster as a result, all four agree to start using their powers to defend the world from evil.
I can well believe that Jack Kirby might have used COTU as a base idea from which to propose some concept that became the Fantastic Four. Perhaps he did this on his own, perhaps it came together in response to the now-legendary order from Marvel publisher Martin Goodman, in which he allegedly commanded Stan Lee to come up with a book like DC's JUSTICE LEAGUE. Other fans have noted the fact that Marvel had no individual superhero features in 1961, so that at that time a duplication of DC's JUSTICE LEAGUE scheme could not have worked. Logically, an original hero-team along the lines of COTU-- and its two or three DC-imitators, like SEA DEVILS and CAVE CARSON-- probably seemed the ideal solution to the problem.
However, there's one big problem with seeing the FANTASTIC FOUR as entirely the spawn of Kirby's CHALLENGERS work: in terms of mythicity, of symbolic resonance, COTU is like an artist's sketch of a fully formed idea, while even in the early, somewhat crude FANTASTIC FOUR stories, the idea has been developed.
The Kirby COTU stories-- some or all of which may've started from full scripts by other writers, though Mark Evanier asserts that Kirby usually rewrote any scripts he was given-- resemble the kind of stuff Kirby did in his mid-to-late 1970s Marvel works. The narratives usually begin with the Challengers being summoned to investigate or eludicate some mystery, usually one with strong SF-aspects, and thereafter the heroes fights aliens or robots or whatever in fast-action stories with no great emotional resonance.
In contrast, even the weakest of the early FANTASTIC FOUR stories show a concern for dramatic storytelling, in which the individual characters have distinct personalities and relationships with one another. Because of such distinctions, the first ten FF stories sustain a mythology of characters, each with his or her archetypal nature: even a pallid villain like the Miracle Man from FF #3 carries some of the resonance of the Faustian deceiver. In contrast, one Kirby robot or alien is pretty much like every other Kirby robot or alien in COTU.
I can't say with certainty that Stan Lee made all the difference. It's certainly possible that Kirby simply had a creative breakthrough with FANTASTIC FOUR like nothing since his 1940s work, and that Stan simply abetted that breakthrough.
But something about that partnership changed the stakes for comic books from then on. I hope in future essays to say something more about it.