Preheat the oven to 350ºF.
Line a 9-inch loaf pan with parchment paper then grease the parchment paper with cooking spray.
In a large bowl, whisk together the flour, sugar, baking soda and salt.
In a separate medium bowl, whisk together the mashed bananas, buttermilk, eggs, melted butter and vanilla. Stir the banana mixture into the dry ingredients.
Shred the zucchini on a box grater. Measure out 1 1/2 cups of shredded zucchini then squeeze is with your hands over the sink to remove excess liquid. Fold the zucchini into the batter then scrape the batter into the prepared loaf pan. Bake the bread for 50 to 60 minutes until a toothpick inserted comes out clean. Transfer the pan to a cooling rack to cool then lift the loaf out of the pan, slice and serve.