He was the pilot on Doolittle Raiders Crew #6.
He was executed in China by the Japanese while a POW.
He was cremated and his remains were stored in Shanghai where they were found after the war and returned to the US for burial in Arlington in 1949.

His DFC citation:
Awarded posthumously for actions during World War II
The President of the United States of America, authorized by Act of Congress, July 2, 1926, takes pride in presenting the Distinguished Flying Cross (Posthumously) to First Lieutenant (Air Corps) Dean Edward Hallmark (ASN: 0-421081), United States Army Air Forces, for extraordinary achievement as Pilot of a B-25 Bomber of the 1st Special Aviation Project (Doolittle Raider Force), while participating in a highly destructive raid on the Japanese mainland on 18 April 1942. Lieutenant Hallmark with 79 other officers and enlisted men 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 himself and the military service.

Action Date: April 18, 1942

Service: Army Air Forces

Rank: First Lieutenant

Company: 1st Special Aviation Project

Division: Doolittle Tokyo Raider Force

Crew #6 (Plane 40-2298, "Green Hornet", target Tokyo). 95th Bomb Sq. L-R: Lt. Chase Jay Nielsen (navigator), Lt. Dean E. Hallmark (pilot), Sgt. Donald E. Fitzmaurice (engineer-gunner), Lt. Robert J. Meder (co-pilot), Sgt. William J Dieter (bombardier). (USAF photo)

Hallmark, Dean Edward, 1st Lt