When the Germans are setting up for the train to arrive at Milan station, they set up mortars and a machine gun that is quite clearly a US .30 caliber water cooled machine gun. The Germans would have been using MG-42's, a much superior weapon and one made by their own country, not the USA.
Mia Farrow was introduced to Frank Sinatra when she visited her friend John Leyton on the set. Farrow had co-starred with Leyton earlier that year when the actress made her screen debut in another British military drama, "Guns at Batasi." She and Sinatra later married.
The three German warplanes in the film, Bf-108s substituting for Bf-109s, sport a spurious, fanciful paint scheme. Luftwaffe service aircraft in most European theaters, including Italian, were camouflaged with a subtle green and brown "splinter" pattern on upper and side surfaces with pale blue undersides.
According to Saul David's memoirs, Frank Sinatra was desperate to have Richard Burton as his co-star. Sinatra was not aware, however, that the studio, 20th Century Fox, were in the middle of a bitter court case with Burton and his wife, Elizabeth Taylor, over the massive cost overruns on " Cleopatra ", and wouldn't even entertain the thought of hiring Burton. Sinatra had made plenty of overtures to Burton in the hope he would sign on, and he was furious that he had wasted his time and effort.
The trivia item below may give away important plot points.
Frank Sinatra mandated that this movie's ending be changed, and subsequently this destroyed any possibility of a sequel that the 20th Century Fox studio was considering. In the original ending which appears in the source novel, Ryan is not killed as with the film's ending; he survives and with the other escaped POWs, is able to get to neutral Switzerland. This ending would have allowed Ryan, the movie's central character, to appear in a sequel. Sinatra wished for his character to be killed off to add a token of believability to the movie, as well as for there to be redemption for the death of the female character Gabriella.