Preheat oven to 350°F (175°C) and grease a 9x5-inch loaf pan.

In a large mixing bowl, mash the bananas until smooth.

Add the cake mix, eggs, vegetable oil, and cinnamon to the mashed bananas.

Stir the mixture until just combined; do not overmix.

Pour the batter into the prepared loaf pan.

Bake for 45 to 55 minutes, or until a toothpick inserted into the center comes out clean.

Allow the bread to cool in the pan for 10 minutes before transferring to a wire rack to cool completely.