did you check the Wikipedia page on Superman about the influences?
The word "superman" was commonly used in the 1920s and 1930s to describe men of great ability, most often athletes and politicians. It occasionally appeared in pulp fiction stories as well, such as "The Superman of Dr. Jukes". It is unclear whether Siegel and Shuster were influenced by Friedrich Nietzsche's concept of the Übermensch; they never acknowledged as much.