Dean Hallmark was born on January 20, 1914, in Robert Lee, Texas. He attended Auburn University before becoming a civilian pilot before World War II, later flying petroleum workers around in Venezuela in 1939 and 1940. Hallmark enlisted in the Aviation Cadet Program of the U.S. Army Air Corps on November 21, 1940, and was commissioned a 2d Lt and awarded his pilot wings at Stockton, California, in July 1941. His first assignment was as a B-25 Mitchell pilot with the 95th Bomb Squadron of the 17th Bomb Group at Pendleton Field, Oregon, from July 1941 until he was selected for the Doolittle Mission in February 1942. During this time, he and his squadron flew anti-submarine patrols off the West Coast of the United States. Lt Hallmark was the pilot on the sixth B-25 to take off from the aircraft carrier USS Hornet (CV-8) on April 18, 1942, and after bombing targets in Tokyo, his crew flew to China and ditched the plane off the coast and then swam to shore. He was captured by the Japanese on April 27, 1942, and after a mock trial was executed on October 15, 1942. After the war his remains were recovered and buried at Arlington National Cemetery in January 1949.

His Distinguished Flying Cross Citation reads:

For extraordinary achievement while participating in a highly destructive raid on the Japanese mainland on April 18, 1942. 1st Lieutenant Hallmark volunteered for this mission knowing full well that the chances of survival were extremely remote, and executed his part in it with great skill and daring. This achievement reflects high credit on Lieutenant Hallmark and the military service.