Robert Montgomery was a real-life PT skipper in World War 2. He helped direct some of the PT sequences for the film after John Ford broke his leg three weeks into filming. Montgomery finished the film and was complimented by Ford for his work. Ford claimed he couldn't tell the difference between his footage and Montgomery's, who took no screen credit.
By the time the film was finished the Japanese had surrendered, so MGM pushed the release date back to December 1945. With the war over, the film opened to enthusiastic reviews but low turnout at the box office. As John Wayne later said, "People had seen eight million war stories by the time the picture came out, and they were tired of them."
Robert Montgomery was able to draw on his activity as an PT commander (at Guadalcanal and Normandy), as could James Curtis Havens, one of the second unit directors and the film's explosives expert.
Following the end of WW2 most of the PT boats remaining overseas were burned to save the expense of bringing them home. Since they were made of plywood, rather than metal, they were not considered to be useful for anything. They were stripped of engines and armament and then torched. Only a few that were still in the US escaped destruction.
Lindsay Anderson tells the following anecdote in his biography of John Ford: when he interviewed Ford in 1950, the latter admitted he did not like Fuimos los sacrificados (1945)... and actually never saw the finished movie! He disliked everything - project, shooting, editing without his supervision, music added without his consent, etc. Anderson was surprised because he thought it was a good movie, so Ford told him he might watch it after all...
A few weeks later, Anderson received the following telegram that he kept as a memento: "Saw 'They Were Expendable'. You were right. Ford."