Douglas was married twice, but his first marriage to Diana Dill took place on 2nd November 1943. They had two sons together; Michael Douglas and Joel Douglas.
The pair first met when they studied acting together before World War 2. While the war was taking place, Douglas was in the US Navy and he saw a picture of Dill on 3rd May 1943 edition of Life magazine and told his shipmates that would be the woman he would marry. They don't make romance like that anymore!
The couple unfortunately divorced in 1951.
Source: Wikipedia and IMDB.