The Hollywood veteran was assigned to a warship in the Pacific, with strict instructions to track down and explode subs lurking in the deep.
The ruthless nature of the mission set Douglas thinking about the true horrors of war - and the symbolic killing of a bird during a training exercise made it even more harrowing.
He writes on his official blog, "I enlisted in the Navy during World War II. In the Pacific, our assignment was looking for Japanese submarines. It bothered me to imagine young Japanese sailors deep in the ocean crowded on a submarine. They were looking to kill us, we were looking to kill them.
"I wondered if they felt any of the reluctance that I felt. My consolation was that I was better off than an army soldier having personal contact with the enemy. But it's a strange experience from the war that still haunts me.
"It was a sunny day in calm Pacific waters. Everything was quiet except for the constant ping, ping, ping of our underwater radar device searching for submarines. If an echo came back, bouncing off of a submarine, we would try to lower explosives and destroy the submarine and all the young Japanese sailors. That was not a pleasant thought for me.
"Our young captain decided to relieve the boredom of the day with gunnery practice. The command came to man the machine guns - the targets were the graceful swooping gulls circling our vessel. The shooting began and the graceful gulls alluded the bullets flying at them. But, suddenly, one gull, wings outstretched, was hit. All I saw were two lifeless wings slowly falling down into the sea. That terrible vision has never left me."