The Marvel Comics Group began in 1939 (not under that name) and published zillions of comic books. It was not, however, until around 1966 that they made any real money doing anything besides publishing comic books. For long stretches of time in there, they didn't have many continuing characters who could be licensed or merchandised. One of the very few outside deals they made was in 1944 when publisher Martin Goodman licensed Republic Studios to make a fifteen-chapter serial of Captain America. Wikipedia notes…
It stands as the first theatrical release connected to a Marvel character; the next theatrical release featuring a Marvel hero would not occur for more than 40 years. It was the last live-action rendition of a Marvel character in any medium until Spider-Man appeared in the Spidey Super Stories segment of the children's television series The Electric Company in 1974.
The serial was reportedly the most expensive one Republic ever made but apparently almost none of that money went to Mr. Goodman's company. Joe Simon, the co-creator of the character along with Jack Kirby, told me Goodman made the deal for "almost nothing" just to get one of his characters on the screen. The deal also apparently did not include any requirement that Republic adhere to anything in the comic books or even mention in the titles that there were Captain America comic books. They were though mentioned in some of the advertising.
Those who know more about serials than I do have said that the script was not even written with Captain America in mind. There are several theories as to who the hero of the serial was originally intended to be but it does not seem to be in dispute that Republic couldn't close the deal for some other property they wanted. Thus, the script was switched to Captain America because Goodman was so eager to sell. Captain America in it is not Steve Rogers the super-soldier. He's District Attorney Grant Gardner and he uses a gun, not a shield.
I know some folks love these old serials but I find them unwatchable for more than a chapter or so. Someone has posted all fifteen chapters to YouTube and I've embedded Chapter One below. If you enjoy it, it shouldn't be hard to find and watch the other fourteen installments but I doubt a lot of you will. Neither Simon nor Kirby ever watched the whole thing…