Besides, the P-51s were purchased by the U.S. government. The planes belonged to them and its not like an image of a warplane should ever become a trademark.
North American built a number of iconic aircraft. Which one would be a trademark?
North American Aviation (NAA) was a major American aerospace manufacturer, responsible for a number of historic aircraft, including the T-6 Texan trainer, the P-51 Mustang fighter, the B-25 Mitchell bomber, the F-86 Sabre jet fighter, the X-15 rocket plane, and the XB-70, as well as Apollo Command and Service Module, the second stage of the Saturn V rocket, the Space Shuttle orbiter and the B-1 Lancer.