The "Untold Origin" of
 
DC Special No.29
The most sensational secret 
of WWII
Thirty-six years after their first meeting, the Secret Origin of the Justice Society of America was finally revealed in DC Special No.29 (Cover date: September 1977).  Written by Paul Levitz, The cover was by Neal Adams and the artwork by Joe Staton and Bob Layton.  

The heroes stop the invasion of the United Kingdom and the assassination of FDR by Hitler and his mystic Valkyries.  They then decide to join forces to combat the forces of evil. 

 
Secret Origins no.31 
The origin was revised in October 1988 to fit the post- Crisis on Infinite Earths DC Universe as part of the on-going Secret Origins series.  (Taking out Batman and Superman - the later resulting in a major plot alteration.) Script by Roy Thomas, art by Michael Bair & Bob Downs.  

Thomas also retold the origins of each individual JSAer as part of this informative series.  Well worth a look.