Captain Marvel is not a rip-off of Superman. There are similarities, however, Captain Marvel is very different. He's not an alien from another planet, rocked to Earth, etc. Unlike the extraterrestrial Superman, Captain Marvel is magical, going back to old folklore and mythos of classic times, Greek mythology. The 3,000 year-old wizard Shazam turned young Billy Batson into Captain Marvel with the wisdom of Solomon, the strength of Hercules, the stamina of Atlas, the power of Zeus, the courage of Achilles and the speed of Mercury. The wizard Shazam's name is from the first letters of the seven Elder's names. Billy Batson becomes Captain Marvel by saying the magic word, which is the wizard's name, Shazam. Superman never lost his powers as Clark Kent, but Billy Batson is as vulnerable as any young, non-super-powered boy, thus, more relatable.
Superman's look was modeled after Douglas Fairbanks, as Clark Kent his look was modeled after Harold Lloyd. Captain Marvel's look was modeled after Fred McMurray. The wizard Shazam's look was modeled after Merlin. Superman's costume was modeled after circus/carnival strong man costumes, with trunks over tights. Captain Marvel's costume was modeled after both military uniforms, with a buttoned lapel and boxy shoulders, buccaneer boots, and ancient Egyptian and Persian costumes, with arm bands, a sash with a dangling tail, a cape with braid/button loops that's held around his neck by a cord and hanging over one shoulder was modeled after the ceremonial cape worn by the British nobility.
Captain Marvel was shown flying from Washington to New York and from New York to Chicago in Whiz Comics #5 (May, 1940) "Captain Marvel Crashes Through" by creators Bill Parker and C.C. Beck, while Superman was still only leaping an eighth of a mile.
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Captain Marvel Jr. first appeared in Whiz Comics #25 (December, 1941), years before Superboy first appeared in More Fun Comics #101 (January–February, 1945). Mary Marvel first appeared in Captain Marvel Adventures #18 (December, 1942), years before Supergirl first appeared in Action Comics #252 (May, 1959). Even Hoppy the Marvel Bunny first appeared in Fawcett's Funny Animals #1 (December, 1942), years before Krypto the Superdog first appeared in Adventure Comics #210 (March, 1955). In Richard Donner's Superman: The Movie (1978) and Donner's Superman II (1978), Marlon Brando's Jor-El was a Shazam-like mentor spirit who counsels Superman, like the wizard Shazam counsels Captain Marvel, and in a huge Fortress, that is reminiscent of the the wizard Shazam's Rock of Eternity. Even Luthor's assistant Ms. Teschmacher who has feelings for Superman and saves him, is like Sivana's assistant Beautia who has feelings for Captain Marvel, and she saves him. Beautia first appeared in Whiz Comics #3 (April, 1940) "The Return of Sivana" by Bill Parker and C.C. Beck.