Bake the Cake: Prepare the white cake mix according to the package directions, using egg whites instead of whole eggs to keep the cake as white as possible. Pour the batter into a greased 9x13-inch baking pan and bake according to the box directions.
1 box white cake mix, Ingredients listed on the cake mix box
Cool Slightly: Let the cake cool in the pan for 20 minutes. The cake should still be a little warm so it can absorb the Jell-O well.
Make the Jell-O: In a heat-safe bowl or large measuring cup, whisk the strawberry Jell-O into the boiling water until fully dissolved. Stir in the cold water and mix until combined.
1 cup boiling water, 1 box, 1/2 cup cold water
Poke the Cake: Use a fork to poke holes all over the top of the cake. Make plenty of holes so the strawberry mixture can soak in evenly.
Add the Filling: Slowly pour the Jell-O mixture over the entire cake, letting it run into the holes and across the surface.
Chill the Cake: Refrigerate the cake for 30 minutes so the Jell-O can set and the cake can cool completely.
Top the Cake: Spread the thawed Cool Whip evenly over the top of the chilled cake.
8 ounces Cool Whip
Prep the Berries: Rinse the blueberries and strawberries well. Hull the strawberries and slice them lengthwise.
1 pint blueberries, 1 quart strawberries
Decorate the Flag: Arrange the blueberries in the upper left corner of the cake to create the blue section of the flag. Lay the sliced strawberries in horizontal rows across the rest of the cake to create the red stripes, leaving space between each row for the white stripes.
Chill Again: Refrigerate the decorated cake for at least 1 hour before serving so it slices more cleanly and holds its shape.