Heat oven to 350°F (325°F for dark or nonstick pan).
Make and bake cake mix as directed on the box for a 13x9-inch pan. Cool 10 minutes. Run a knife around the sides of the pan to loosen the cake; remove from the pan to a cooling rack. Cool completely, about 1 hour.
Place cake on a tray. Frost the top and sides of the cake with frosting.
For flag design, arrange blueberries on the upper left corner of the frosted cake to represent stars; arrange strawberries in rows over the frosted cake to create stripes. Serve immediately. Store covered in the refrigerator.
