Detroit became known as the Arsenal of Democracy during World War II, when its factories were rapidly converted to produce armaments for the Allies. The phrase is attributed to President Franklin Delano Roosevelt.

Fort Wayne and the State Fairgrounds served as central hubs for Detroit's war effort. Tanks and equipment were brought there before being shipped overseas.

References

http://apps.detnews.com/apps/history/index.php?id=73&category=locations