The Longest Day is filmed in the style of a docudrama. Beginning in the days leading up to D-Day, the film concentrates on events on both sides of the channel such as the Allies waiting for the break in the poor weather and the anticipation of the Axis forces defending northern France. The film pays particular attention to the decision by General Eisenhower, supreme commander of SHAEF, to go after reviewing the initial bad weather reports as well as the divisions within the German High Command on where an invasion might happen or what response to it should be.
Numerous scenes document the early hours of 6 June when Allied airborne troops were sent in to take key locations. The French resistance is also shown reacting to the news that an invasion has started. The Longest Day chronicles most of the important events surrounding D-Day. From the British glider missions to secure Pegasus Bridge, the counterattacks launched by American paratroopers scattered around Sainte-Mère-Église, the infiltration and sabotage work conducted by the French resistance and SOE agents, and the response by the Wehrmacht to the invasion and the uncertainty to whether it was a feint in preparation for crossings at the Pas de Calais (see Operation Fortitude).
During the filming of the landings at Omaha Beach, the extras appearing as American soldiers did not want to jump off the landing craft into the water because they thought it would be too cold. Robert Mitchum, who played General Norman Cota, became disgusted with their trepidation. He jumped in first, at which point the extras followed his example.
The Rupert paradummies used in the film were far more elaborate and lifelike than those actually used for the decoy parachute drop (Operation Titanic), which were actually just canvas or burlap sacks filled with sand. In the real operation, six Special Air Service soldiers jumped with the dummies and played recordings of loud battle noises to distract the Germans.
In the scenes where the paratroopers land, the background noise of frogs croaking "ribbit ribbit" was wrong for northern French frog species and showed that the film probably used an American recording of background night noises.
In the film, 3 Free FrenchSpecial Air Service paratroopers jumped into France before British and American airborne landings. This is accurate. 36 Free French SAS (4 sticks) jumped into Brittany (Plumelec and Duault) on June 5 (11 h 30). The first Allied soldier killed in action was Lieutenant Den Brotheridge of the 2nd Ox & Bucks Light Infantry as he crossed Pegasus Bridge at 0h 22m on 6th June.
The United States Sixth Fleet extensively supported the filming and made available many amphibious landing ships and craft for scenes filmed in Corsica, though many of the ships were of (then) modern vintage. The USS Springfield, and USS Little Rock, both World War II light cruisers (though extensively reconfigured into guided missile cruisers) were used in the shore bombardment scenes, though it was easy to tell they did not resemble their wartime configuration.
Among historical inaccuracies was the calling up of Engineers to "blow a path" through a "sea wall" to allow forward troop movement. The Sea Wall was, at highest, one meter, made of rocks, and lay at the edge of high tide. This barrier was used to prevent erosion and on June 6, 1944, provided cover for the overwhelmed first two waves.
One glaring omission was the almost complete absence in the film of action by the Canadian forces on D-Day, considering they were the only troops to gain their assigned objectives.
Charlton Heston actively sought the role of Lieutenant Colonel Benjamin H. Vandervoort but the last-minute decision of John Wayne to take the role prevented Heston's participation. At 55, Wayne was 28 years older than Vandervoort at the time of action (and 10 years older in real life). While everyone else accepted $25,000 as payment, John Wayne insisted on $250,000 to punish Zanuck for referring to him as "poor John Wayne" regarding Wayne's problems with his lavish movie The Alamo.
Sergeant Kaffeekanne's name is German for "coffee pot", which he always carries.
Richard Todd, who played Major John Howard, leader of the British Airborne assault on the Pegasus Bridge, took part in the real bridge assault on D-Day. Todd was offered the chance to play himself but took the part of Major John Howard instead. In the film, shortly after the British have captured the Orne bridge (later renamed Horsa Bridge), one of the soldiers tells Todd, playing Howard, that all they have to do now is sit tight and await the arrival of the 7th Parachute Battalion, to which Todd's character replies dismissively: "the Paras are always late". This was a private joke, Todd had been the adjutant of the 7th Parachute Battalion on D-Day.
Former US President Dwight D. Eisenhower was considered for the role of himself in the film, and he indicated his willingness. However, it was decided that makeup artists couldn't make him appear young enough to play his World War II self. The role of General Eisenhower went to Henry Grace, a set decorator with no acting experience, but who had been in the film industry since the mid-1930s. He was a dead ringer for the younger Eisenhower, though his voice differed.
Mel Ferrer was originally signed to play the role of General James M. Gavin but withdrew from the role due to a scheduling conflict.
According to the 2001 documentary Cleopatra: The Film That Changed Hollywood, Richard Burton and Roddy McDowall were so bored having not been used for several weeks while filming in Rome, they phoned Zanuck begging to do "anything" on his film. They flew themselves to the location and each did a day's filming for their cameo-performances for free.
The film premièred in France on 25 September 1962, followed by the United States on 4 October and 23 October for the United Kingdom.
Unique for British and American produced World War II films of the time, all French and German characters speak in their own languages with subtitles in English. Another version, which was shot simultaneously, has all the actors speaking their lines in English (this version was used for the film's trailer as all the Germans deliver their lines in English). However this version saw limited use during the initial release. It was used more extensively during a late 1960s re-release of the film.
The English-only version has been featured as an extra on older single disc DVD releases.