In the mid-1960s, thanks to the way that copyright law was set up at the time, Captain America co-creator Joe Simon filed legal paperwork in an opportunity to win back the rights to the Star-Spangled Avenger and the work that he and Jack Kirby and a variety of other hands had performed in the first ten issues of CAPTAIN AMERICA COMICS in the early 1940s. The super hero boom was in full swing, and Captain America was one of the few characters at that point who had lived a life beyond the four color comic book pages and who was likely to be known to a civilian audience.
Marvel founder and owner Martin Goodman wasn’t about to give up his ownership of Captain America without a fight, and he began reprinting the earliest Captain America stories (or the earliest ones for which any reproduction materials could be sourced) in the pages of FANTASY MASTERPIECES, to re-establish his rights to them. He and editor Stan Lee did remove the Joe Simon & Jack Kirby credit form all of these stories as a matter of course, despite the fact that Marvel by then point made it a habit to credit the creators on its titles.
Goodman also prevailed upon Jack Kirby, then working as the backbone of the Marvel creative line, to testify on Marvel’s behalf, indicating that if Simon were to win, the firm might suffer greatly and impact on Kirby’s ability to make a living. This testimony caused no end of headaches for Kirby in later years, as it both predates any later recountings of these and similar events and is at odds with how Kirby would recall these events in later years.
That testimony is a matter of public record, though. So here is Jack Kirby’s testimony on the question of creating Captain America in 1941: