"The Precision Anti-Aircraft Bullet"
Major Thorpe (writer and artist), Mort Leav (pencils and inks)
Published in Jumbo Comics #28, Jun 1941 - 8 pages
In a tiny German village, an inventor named Brecker shows ZX-5 a precision anti-aircraft shell he has perfected which he wants to get to Britain. Unfortunately the Gestapo gets involved.
Note: this is the first mention in the series of Nazis and the Gestapo specifically