Born on October 31, 1918, in Aurora, Illinois, Truemper was one of 10 children. He attended business college and worked as an accounting clerk before enlisting in the United States Army from Aurora on June 23, 1942. He served for a few months with the 174th Field Artillery at Camp Bowie, Texas, until he entered flying training. He took pre-flight training at Ellington Field, Texas; flexible gunnery at Harlingen, Texas; and advanced navigation at Hondo, Texas, and was commissioned a second lieutenant, Air Corps on August 26, 1943. He served with the 796th Bomb Squadron at Alexandria, Louisiana, until being sent to the Eighth Air Force as a replacement aircrew member in December 1943. He was assigned to the 510th Bomb Squadron, 351st Bombardment Group, based at RAF Polebrook in England.