From the estate of the American artist James Lunnon, this is an unsigned proposed Black Mask pulp cover painting which was slated for publication in May of 1940. The yellow left side field is a telling trait and was unique to the title - this format was used between January 1937 to April of 1940, with many of these covers painted by James Lunnon. There was no May, 1940 issue, as the title was sold to Popular Publications right after the April 1940 issue and when the magazine appeared again in June of 1940, it had a completely redesigned look. Our guess is that this painting was intended as the cover for the 5/40 issue, but with the change of publishers causing that issue to be skipped, and the then resulting change in cover design, it was never used.
For comparison purposes here are three Black Mask covers by James Lunnon.