This is the first issue with a DC bullet on cover.
The on-sale date is the publication date reported in the U. S. Copyright Office filing in the Catalog of Copyright Entries, Part 2, Periodicals, New Series, Volume 35, 1940, Number 2. Second class permit. Copyright number 449540. The filing states that Detective Comics, Inc. is the publisher and copyright holder.
First mention of Daily Planet in comic book.
Superman #4 went on sale 2/15/40, which predates Action 23. However, careful examination of the two Luthor stories in Superman #4 and the Luthor story here, it becomes clear that this Luthor story in Action #23 comes first. One telltale clue is to see Superman asking someone "Who is Luthor?"
The last panel of this story is an illustrated promo ad for the Sandman in Adventure Comics.
Title is given in panel 3, page 1 of story.
Writer credit added and art verification by Craig Delich 8-22-08.
Letterer credit added by Craig Delich 8-22-08.