Planet Comics ran for 73 issues between 1940 and 1954, and sported consistently amazing covers. You can read all 73 for free online at comicbookplus.com (right HERE) (Thanks to Mike Britt for the tip), and you can peruse all the covers in Todd Frye's new book Jungle Queens and Space Rangers. We present here but a few.
Day before day before yesterday: FIGHT COMICS
Day before yesterday: JUMBO COMICS
Yesterday: JUNGLE COMICS
Tomorrow: WINGS COMICS