That's a simply smashing cover by Don Heck for the debut issue of Mandrake the Magician from the brief King Features brand. Below is that artwork in its original form, and Heck's deft craftsmanship is even more evident.
Here's the cover of the first Charlton Flash Gordon by Reed Crandall. It was issue twelve of the run and my first issue of Flash as well.
Here's what Crandall had planned for the comic if it had stayed under the King Features brand. The cover used was converted from an interior panel if I remember correctly.
Two great images by two great pros.