Short answers, pulled from the story.
Arsenal of Democracy refers to the collective efforts of American industry supporting the Allies during the Second World War. Those efforts concentrated in established industrial centers such as Detroit, Cleveland, Philadelphia, Buffalo, Rochester, Chicago, New York, and Pittsburgh.
Roosevelt delivered the Arsenal of Democracy address in a radio broadcast on the 29th of December 1940. That was nearly a year before the United States entered the Second World War and less than a year before the attack on Pearl Harbor on the 7th of December 1941.
Doubleday executive Herbert S. Houston used a version of the phrase in a 1918 article titled 'Blocking New Wars.' Nazi propagandist Joseph Goebbels used 'arsenal of democracy' in his newspaper Der Angriff on the 30th of April 1928, and playwright Robert E. Sherwood spoke of America as an arsenal for the democratic Allies in the New York Times on the 12th of May 1940.
Credit is usually given to Roosevelt's advisor Harry Hopkins. Another account claims Roosevelt borrowed the phrase from Detroit auto executive William S. Knudsen, whom Roosevelt had tapped to lead the country's war material production.
Roosevelt wanted to argue against American isolationism and build support for supplying Britain with military equipment while the United States stayed out of active fighting. He warned that a British defeat would let the Axis powers threaten the Western Hemisphere, invoking what he called the geopolitical Heartland theory.
On the day of the speech, only 458,365 non-Coast Guard personnel served on active duty, split among the Army, Navy, and Marine Corps. Within a year that number had nearly quadrupled to 1,801,101 personnel.