A perfect ripe banana will have many brown spots speckled all over the peel. The peel may also turn completely brown/black. This allows the sugars to form in the banana and adds perfect sweetness to banana bread. If you're in a hurry and only have unripe or yellow bananas, pop them into a 300°F oven (with the peel on) on a lined rimmed tray (they may ooze a little) and bake until they turn black, about 20 minutes depending on how unripe the bananas. Let cool before peeling.