Heat the oven to 350°F and arrange a rack in the middle. Coat a 9-by-5-inch metal loaf pan with butter and flour, tapping out any excess flour; set aside.
Place the measured flour, baking powder, cinnamon, baking soda, nutmeg, and salt in a medium bowl and whisk to aerate and break up any lumps. Set aside.
Place the eggs, sugars, oil, and vanilla in a large bowl and whisk until the eggs are broken up and the mixture is thoroughly combined. Using a rubber spatula, fold in the flour mixture until just combined. Fold in the zucchini until evenly mixed.
Pour the batter into the prepared pan and spread it into an even layer. Bake until a cake tester or toothpick inserted in the center comes out clean, about 1 hour to 1 hour 10 minutes.
Transfer the pan to a wire rack to cool slightly, about 15 minutes. Run a knife around the perimeter of the pan and turn the bread out onto the rack to cool completely.