The stories of Jerry Siegel and the art of Joe Shuster were published in comic books nationwide long before Superman appeared on the cover of and within the pages of Action Comics #1, June 1938. The first published stories by our honored team appeared in New Fun Comics #6, in October, 1935. Siegel and Shuster produced two features for that issue - "Dr. Occult" and "Henri Duval of France, Famed Soldier of France".
Siegel and Shuster produced only five segments (six pages) of Henri Duval set in the time of the Musketeers. Dr. Occult was a bigger success, eventually appearing in 27 issues of New Fun/More Fun Comics (#s 6-32) through June, 1938. Under the name Dr. Mystic he appeared in The Comic Magazine #1 as well for one page. Dr. Occult was a supernatural detective. Among his powers were astral projection, hypnosis, illusion, and telekinesis. His costume was usually a fedora and a trench coat, but for one storyline he wore a skintight body suit with external trunks and a cape. The Henri Duval feature was replaced by a different feature at first called "Calling All Cars". With More Fun #19 the features name was changed to "Radio Squad". The heroes were a pair of partnered policemen who shared a squad car. The last Dr. Occult four page segment appeared in More Fun #32 (June, 1938), the same month as Action #1. Shuster continued to draw Radio Squad six page tales until More Fun #49 (November, 1939). Siegel wrote the series stories through More Fun #70 (August, 1941).
Comic book stories by Siegel and Shuster appeared in other magazines published by the company that became National Comics/DC. While New Fun/More New Fun/More Fun was the first comic they produced, their second was New Comics/New Adventure Comics/Adventure Comics. The team missed the first issue of New Comics, but in issue #2 (January, 1936) Siegel and Shuster told a four page story of "Federal Men". The federal men were G-men. Two to four page segments would appear in New Comics #s 3 to 11. They would continue in New Adventure #s 12 to 31 except for issue # 15. When the title became Adventure, the sequence continued by both Siegel and Shuster through Adventure #36 (March, 1939). Seven more stories with input from other members of the Shuster shop would continue through issue # 43 (October 1939). Siegel would author stories until Adventure #57 (December, 1940).
The third comic book title Siegel and Shuster would contribute to before Superman's origin appeared was the title that would eventually give the company its name. DC stands for Detective Comics. The first issue of Detective Comics hit the stands dated March, 1937. Two different Siegel and Shuster features were premiered in that issue. They were "Slam Bradley" and "Bart Regan, Spy". Of the two, Slam Bradley was the more robust. At 13 pages, it was frequently the longest feature in the book. It was most frequently the last major story, which was a favored, featured position in the anthology titles of that era. Occasionally it would be the lead, but rarely the cover feature. The stories bear some resemblance to Roy Crane's Wash Tubbs comic strip. Slam's partner, the descriptively named Shorty Morgan, bore a height resemblance to Wash, while Slam fulfilled the position of Captain Easy, the rough and tumble man of action. Slam and Shorty would travel the world as detectives, inserting themselves into other peoples' problems and bringing rough justice to solve them. In the mostly four page Spy tales Bart Reagan was mostly a counterspy for the U.S. government, dealing with foreigners and traitors. The last Slam Bradley to bear Shuster's name was the story in Detective # 32 (October, 1939), the last Spy Detective #28 (June, 1939). Siegel was the writer for both through issue #55 (September, 1941). The Slam Bradley feature would run until Detective #152 (October, 1949) as Slam Bradley, but Bart Regan would be dropped from the name of that feature and become just Spy. As Spy, its last issue was #72 (February, 1943).
Thus by the time Superman appeared in Action #1 in June of 1938, the team were very experienced comic book creators. In that month besides Superman they had five features appearing in DC comics:
Dr. Occult
Radio Squad
Federal Men
Slam Bradley
Spy
The difference between those features and Superman was the long term success and influence of the latter compared to the others. They had been in the business for over two years, almost as long as anyone else creating comic books at the time. Unfortunately they were inexperienced business men.