On 11 June 1939, the New York Times printed the menus for all of the meals served to the British royals during their visit to Hyde Park. As depicted in this movie, the picnic menu was as follows: Hot dogs, if weather permits; beer and soft drinks; cold ham from various states; turkey, smoked and plain; various salads; doughnuts; ginger bread cookies; coffee; and baked beans & brown bread.
The make and model of the real-life car that President Franklin D. Roosevelt drove with the special hand controls was a 1936 Ford Phaeton, and it is on display at the "FDR Presidential Library & Museum" in Hyde Park, New York State, USA.
The gag about the antique anti-U.K. political cartoons is based on truth. These historic cartoons are located in the foyer of Franklin D. Roosevelt's home, King George VI viewed them them upon arriving in the foyer, and made a brief humorous remark to FDR about the cartoons.
Even though it is a well known historical fact that both Franklin D. Roosevelt and King George VI were heavy smokers, the film makers had to get official permission to show the characters smoking on screen.
The entire film was shot in England.