|Copyright MARVEL COMICS
1) There are four members in each group.
2) Both groups survived plummeting from the sky.
3) Both strips were drawn by Jack Kirby.
From my point of view, the first two are superficial and are outweighed by the differences, which are...
1) The FF had superpowers, the Challengers didn't.
2) One of the FF was a woman, whereas the Challengers were all men.
3) The Challengers survived a plane crash which should have killed them, whereas the FF's rocket returned safely (if roughly) to earth once the automatic pilot took over.
4) The Challengers didn't know one another before the crash, whereas the FF were friends before undertaking their space flight.
Also, it was surviving a crash which should've killed them that motivated the Challs into undertaking their adventures; with the FF it was being transformed by cosmic ray-endowed superpowers which inspired them to band together as a team - not quite the same thing. There might be other similarities and differences, but that's enough to work with for now. My point being that I think any comparisons made between the FF and the Challengers in an attempt to ascribe total creative authorship to the same man are exaggerated and don't stand up to scrutiny.
Also, the FF came about because my publisher, Martin Goodman, tasked me with creating a group that would cash-in on the success of DC's Justice League Of America, which was proving to be a popular hit at the time, so there was a specific reason as to why they came to be. The FF mag wasn't just some casual creation, randomly churned out on a whim to see what might stick - it was a specific response intended to capture a slice of the relatively recent re-emerging market for comicbook superheroes.
The fact that a re-envisioned Human Torch was part of the group suggests that Martin Goodman (or myself) might have at first wanted to use some of our back catalogue of Timely's heroes. It may well be that Captain America (maybe Bucky as well) and Sub-Mariner were among those considered for inclusion before I (possibly with some input from Jack, possibly not) came up with the line-up as seen in the published mag. There had already been an attempt to revive the aforementioned wartime heroes only a few short years before which hadn't been the success we'd hoped for, hence, possibly, that idea being abandoned.
My synopsis (below - click to enlarge) for the first issue still exists (no surprise that some of my critics doubt its authenticity), but I simply can't recall whether or not I'd already talked things over with Jack before typing it. However, there are aspects of it that Jack changed at the drawing stage, and the Mole Man section may well have been entirely down to the Jolly one. Though again, we might have discussed it at some point prior to Jack drawing it, just can't remember. Things like that just didn't seem important at the time - we were all too busy doing it to dwell on it.
However, once the series was well under way, Jack assumed a more creative input into the plots with only minimal 'interference' from me, though I'd say my editing, scripting, and characterisation, along with occasional plot tweaks, were essential elements in why the mag became so popular. That's why, when divvying up the credits, it's in no way a disservice to either of us to regard myself and Jack as co-creators of 'The World's Greatest Comic Magazine!'
'Cos we were, and it was!