Preheat your oven to 350°F (175°C) and grease your loaf pan.
In a separate bowl, whisk together the flour, baking soda, salt, and cinnamon (if using).
In a large bowl, combine the mashed bananas, pineapple, melted butter, sugar, eggs, and vanilla extract. Stir until well combined.
Slowly add the dry ingredients to the wet ingredients, stirring gently until just combined. Be careful not to overmix the batter, as this can make the bread dense.
Pour the batter into the prepared loaf pan and smooth the top.
Bake for 50-60 minutes, or until a toothpick inserted into the center comes out clean or with a few moist crumbs.
Let the bread cool in the pan for about 10 minutes, then transfer it to a wire rack to cool completely.