The best way to quickly ripen bananas for eating or slicing is in a paper bag. If you add one or two well-ripened banana or apples to the unripe bananas, the process will go much more quickly. The ethylene gases from the ripened fruit will hasten the ripening. The bananas should be ripe enough to eat in a day or two.
For banana bread or cake, put the unpeeled bananas on a foil-lined baking sheet. Bake at 350° F for 15 to 20 minutes, or until the bananas are completely blackened. Let them cool, scoop the banana from the peel, and mash for banana bread.
The microwave oven is another way to ripen bananas for banana bread or other recipes. Peel the bananas and place them on a microwave safe plate or bowl. Heat just until you hear them sizzling, about 30 seconds.