Groucho Marx offered the following explanation for the movie's title: "Take two turkeys, one goose, four cabbages, but no duck, and mix them together. After one taste, you'll duck soup the rest of your life."
Shortly before this film premiered, the city of Fredonia, New York, complained about the use of its name with an additional "e". The Marx Brothers' response was, "Change the name of your town, it's hurting our picture."
An early draft of the script introduces Freedonia's new dictator Rufus T. Firefly as an agent for an ammunition company, which led to lots of ammunition-salesman jokes and intertwined with the movie's war theme.
According to The Marx Brothers biographer Joe Adamson, the elaborate "All God's Chillun Got Guns" musical number was mostly improvised on the set, as there is no reference to it in the movie's final script. It's likely Groucho Marx was referring to the old Negro spiritual "I got Shoes", which repeats the line "All of God's children got..." filling "a song", "a robe", "a harp", etc. Groucho just added "Guns".
Harpo Marx's character was originally called Skippy, but this was changed to Pinky after his role in Horse Feathers (1932), making him one of only two of The Marx Brothers to reuse a character name in their films (not counting when they used their own names).
Harpo Marx's character Pinky went through several name changes in pre-production. In a 1933 Paramount press-book ad, he's known as Snoopy. For a radio trailer, his character's name was changed to Skippy.