One comic book website attributes the 1st time travel story in comic books to be Slam Bang Comics #1(Fawcette Pubs Jan 1940). This book kicked off the series called "Mark Swift and his Time Retarder". It tells the tale of grade school teacher Rodney Kent and his adopted son Mark inventing their own time machine. It reminds me of that great Rocky and Bullwinkle feature "Prof Peabody and Sherman and their Way-Back Machine"
Originally Posted by Phil Maurice
Anyone have an earlier example?