Most Star Spangled Banners you'll see on July 4 were made in the USA. Annin Flagmakers, the U.S.'s oldest and largest flag manufacturer, produces more than 3 million full-size American flags yearly.

Outside, Sew Valley’s West End building is covered with graffiti. Inside, there are mannequins, sewing machines and a large whiteboard with a goal written on it: 3,750 flags. But more important than ...