I was inspired by this to ask a question I've had for a while:
It what issue(s) did Batman first lose the hyphen in his name? Have any of the writers stated why they changed the spelling?
Batman started off as "The Bat-Man" and "The Batman", simultaneously.
The cover of Detective Comics No. 27 (which contained the first Batman story, "The Case of the Chemical Syndicate") says THE BATMAN while the interior says THE BAT-MAN:
By April 1940, Batman had his own comic, the eponymous Batman, with no hyphen in the title:
Superman was introduced in Action Comics No. 1 as "Superman" with no hyphen:
Given Superman's rapidly ascending popularity, followed quickly by the introduction of popular DC hero Aquaman (More Fun Comics No. 73, 1941), it makes sense for DC to have consolidated its hero names as single words with no hyphens, using Superman as a model especially.
Of course, Wonder Woman has always been two words (mainly because "Wonderwoman" looks awkward), but it's worth noting that her name lacks hyphenation. Given that she was also introduced in 1941, this lends further support to DC galvanzing its naming strategy in the early 1940s, at least as far as their main heroes were concerned.