Wonder Comics #1 (May, 1939), featuring the first - and only - appearance of the original Wonder Man.
In 1939, just a year after his debut, Superman was so popular that he was practically printing money for National/DC Comics. Magazine publisher Victor Fox saw how much money DC was raking in (some sources state Fox had been one of DC's accountants, but comics historian Gerard Jones says that's not true) and decided to get into the comic book business himself. He even opened the office for his new company, Fox Features, in the same building that housed DC.
Fox then contracted Will Eisner and the crew at Iger Studios to create a copy of Superman for Fox's comic books. Eisner created, wrote and drew Wonder Man, which was then published six weeks later in Wonder Comics #1 (a book which also featured some work by a fellow named Bob Kane).
Unlike Superman, "timid radio engineer and inventor" Fred Carson gained his powers from a mysterious yogi (a popular source of super-powers back in the day). The yogi gave Carson a ring that granted him super-strength and invulnerability. Wonder Man could lift tremendous weights, was impervious to bullets and bombs, and could leap several hundred yards in a single bound. Sound like anyone familiar?
DC certainly thought so. They immediately slapped a lawsuit on Fox, claiming that Wonder Man infringed on the Superman copyright. Fox stopped publishing Wonder Man right away, but fought the lawsuit in court.
During the trial, Eisner was called to testify. For years afterward, Eisner claimed that his testimony essentially boiled down to admitting that Wonder Man was created, at Fox's direction, to be a copy of Superman. However, a transcript of Eisner's actual testimony was unearthed in 2010. In that transcript Eisner defends the originality of Wonder Man. In fact, he claimed Wonder Man was his own idea, and had been created years before Superman first saw print in 1938.
(Ironically, Eisner was one of the comic book editors who had rejected Jerry Siegel's and Joe Shuster's Superman strip when they were shopping it around to different companies before finding a home at DC.)
Nevertheless, the court sided with DC. Fox was forced to cease publication of Wonder Man, and the character was not be used in any media at all. It was a first-of-its-kind case for the comic industry, and empowered DC to vigorously pursue action against any other companies that fielded characters that seemed too similar to Superman.
Which eventually lead to the years-long battle between DC and Fawcett over Captain Marvel, but that's a story for another time.
