This has been bandied about quite a bit lately, so I went digging around to see what I could find. It wasn't difficult once I found a good combination of keywords.
The long and short of it is the inspiration for Superman originally came from silent film action stars and the roles they most frequently performed.
The messianic aspect that so many commentators have pointed to, including myself, is more a result of Jung's "collective unconscious" than a deliberate use of metaphor.
Sometimes being wrong is kind of interesting.
My author page at Amazon: https://www.amazon.com/-/e/B000APIPZC?ref_=pe_584750_33951330