17. The Character Wasn't Fully Formed Until The Late 1940s
When you look at a lot of Marvel characters, many of their most defining traits were introduced almost exactly the same time they first appeared. Things have obviously been added and changed over the years, but the basic blueprint was always there. Batman however took quite a bit of time to become anything remotely close to what he is now. Gotham City wasn't introduced until 1940; the now iconic Bat-signal didn't shine until 1942; the Batcave wasn't unearthed until 1948 (before then, Batman had been shown to just have a hangar in a deserted barn). This decade introduced a lot of what makes him so iconic.