Preheat oven to 350°F and spray a 9×5-inch loaf pan.
Grate zucchini and squeeze out excess water with a clean towel.
In a bowl, whisk flour, baking soda, baking powder, cinnamon, and salt.
In another bowl, mash bananas. Add sugar, eggs, oil, and vanilla. Stir well.
Add flour mixture to banana mixture and stir until just combined.
Fold in zucchini until evenly distributed.
Pour batter into the pan and smooth the top.
Bake for 70–80 minutes or until a toothpick comes out with dry crumbs.
Tent with foil during the last 10–15 minutes if browning too quickly.
Cool slightly in pan, then fully on a rack before slicing.
