I could not find any enlistment record for Horton K. Stewart on the NARA AAD site. From his Record of Combat Experience document and from mission load lists, it is clear that he began flying with the 500th Bomb Squadron, 345th Bomb Group while the group was stationed on Biak, Dutch New Guinea.
Lt. Stewart flew seven missions as a copilot beginning on August 21, 1944. On his eighth mission, he was first pilot and continued in that crew position for the remainder of his 41 recorded flights. His Record of Combat Experience credits him with forty combat missions that earned him points used to calculate when his tour was finished. His name was found on one additional flight in the load lists, and that has also been included in the map.