checking out the origin of 'Hugh Hazard and His Iron Man' ( not at all bad for the period) I find a police commissioner firing a flare into the sky to alert the hero. In August 1939, does this predate the Bat Signal?
Just had to add this.
Only serious source of info on George Brenner.
Most sources, which are brief, credit his character 'The clock' as the first masked comic hero. This source is more accurate and gives more detail.
http://qualitycomics.blogspot.com.au/2014/04/george-brenner-full-story.html
But seriously, check the link and tell me, 'Did George Brenner look like ARNOLD or not?'
Link to the book:
Smash Comics 01