Heller was born on May 1, 1923 in Coney Island in Brooklyn, New York, the son of poor Jewish parents, Lena and Isaac Donald Heller, from Russia. Even as a child, he loved to write; as a teenager, he wrote a story about the Russian invasion of Finland and sent it to the New York Daily News, which rejected it. After graduating from Abraham Lincoln High School in 1941, Heller spent the next year working as a blacksmith's apprentice, a messenger boy, and a filing clerk. In 1942, at age 19, he joined the U.S. Army Air Corps. Two years later he was sent to the Italian Front, where he flew 60 combat missions as a B-25 bombardier. His unit was the 488th Bombardment Squadron, 340th Bomb Group, 12th Air Force. Heller later remembered the war as "fun in the beginning ... You got the feeling that there was something glorious about it." On his return home he "felt like a hero ... People think it quite remarkable that I was in combat in an airplane and I flew sixty missions even though I tell them that the missions were largely milk runs" ("Milk runs" were those combat missions that had been mostly uneventful due to a lack of opposition from enemy anti-aircraft artillery or fighters).