Captain Marvel—the world's mightiest mortal—gave Superman many sleepless nights after his debut in Whiz Comics in 1940, two years after the Man of Steel hurtled to planet earth in Action Comics #1. During the 1940s Captain Marvel came perilously close to wresting the ‘most popular superhero’ title from Superman, till DC Comics sued the mighty mortal’s publisher, Fawcett Publications, for copyright infringement.
Fawcett stopped publishing Captain Marvel in 1953 and, in fact, sold the rights to DC which relaunched him in February 1973 in a comic book titled Shazam! The Original Captain Marvel—where he is introduced by none other than Superman himself (see above).
By the time Captain Marvel was integrated into the DC Universe, his appeal and popularity had already waned, even though DC tried to revive him again in 1994.
While Clark Kent pulls off his glasses and rips open his shirt on the run and becomes Superman, Billy Batson merely shouts the magic word “Shazam!” and turns into Captain Marvel.
The only similarities between the two invincible superheroes, as far as I can see, are that both Kent and Batson are reporters—one news, the other radio; as Superman and Captain Marvel they dress and look alike—one in blue, the other in red; and both can fly. Copyright infringement, eh?
Psst!: Whenever young Billy Batson cries out the name of the wizard, Shazam, he is instantly bestowed with the powers of six ancient gods and mythical heroes—Solomon, Hercules, Atlas, Zeus, Achilles and Mercury—the first letters adding up to, you guessed it, Shazam! Powers equivalent to the combined powers of all the superheroes known to you and me. No wonder Superman felt threatened. One more thing: Shazam is not Captain Marvel, it's only a war cry that brings the mightiest mortal alive.