CakeMixDoctor.com has a recipe that uses 3 large, overripe bananas and a box of yellow cake mix to create a moist, simple banana cake. This recipe makes about 12 servings and takes just over an hour to make.
Place a rack in the center of the oven and preheat to 350 F. Grease and flour two 9-inch round cake pans.
Pour one box of yellow cake mix into a large mixing bowl or stand mixer bowl along with 5 tablespoons of flour (omit flour if using Cake Mix Doctor's yellow mix), 3 overripe and mashed bananas,1/2 cup water, 1/2 cup vegetable oil, 3 large eggs, 1 teaspoon pure vanilla extract and 1 teaspoon ground cinnamon. Beat with an electric mixer on low speed for about 30 seconds, until all ingredients are incorporated. Increase the mixer speed to medium and beat until the mixture is smooth, about 1 1/2 minutes, and scrape down the sides of the bowl. Split the batter equally between the prepared pans. Tap out air bubbles and smooth the batter down with a rubber spatula. Bake the cake until it is golden brown and the top springs back when lightly pressed, about 30 minutes. Cool the layers in the pan for 10 minutes before turning out onto wire racks to cool for an additional 30 minutes.
The site suggests cream cheese frosting.