Prepare yellow cake mix according to package directions.
Mash the bananas with the baking soda and stir into the cake mix batter. Pour batter into one greased and floured 9x13 inch baking pan.
Do not preheat oven. Place uncooked cake in oven and turn oven on to 350 degrees F (175 degrees C). Bake for 1 hour. Let cake cool thoroughly and then refrigerate.
