On-sale date from copyright registration. An on-sale date of July 20 was listed in a promotional house advertisement that appeared in Superman (DC, 1939 series) #6 (September-October 1940).
This issue has variants:
Learning the Joker is hospitalized with wounds, the members of the Crime Syndicate make plans to rescue him to lead their mob again. But the Joker has planned a double-cross and the appearance of the Cat-Woman complicates matters.
Batman [Bruce Wayne]; Robin [Dick Grayson]; Professor Drake (death); Goliath (prehistoric man, death); pygmies (villains, some die); Hackett (villain, circus owner, introduction); Snead (villain, circus owner, introduction); Grimes (villain, introduction, death)
Professor Drake brings back from Africa a "missing link" whom he intends to civilize, for the benefit of all civilization. But he learns, as do the Caped Crusaders, that sometimes men act more like beasts than someone like Goliath!