After retraining to fly a North American P-51 Mustang, Wetmore achieved the title of ace, shooting down two Bf 109s on May 19, 1944. He flew an aircraft bearing the legend "Daddy's Girl". There were several planes with this title in the 359th Fighter Group at that time. All of Wetmore's fighters – a P-47D, a P-51B and a P-51D – bore this name. By the end of May 1944, the number of Wetmore's victories had reached 8.25. In a little over a year of military activities, Wetmore had shot down 15 enemy aircraft and been promoted to the rank of captain.