If you have a day or two, you can ripen bananas by placing them in a paper bag with other fruit like apples or more bananas. Fold the top of the bag closed and leave it out on the countertop. The fruit release ethylene, a plant hormone that aids in the ripening process. To ripen bananas even faster, you can bake them in the oven at 300°F for 30-40 minutes or in an air fryer at 300°F for 8-9 minutes until the skins are blackened.