Bake cake according to box directions in a greased and floured 13 x 9" pan. Cool completely. Mix Jell-O and water; then add soda pop. Cool completely. Using the handle of a wooden spoon, poke holes in cake and pour Jell-O mixture over entire cake being sure it goes in all the holes. Slice bananas and cover entire cake. Mix pudding mix with milk. Let thicken slightly. Fold in Cool Whip and spread over cake. Refrigerate.