Grease the inside of a 4-6 quart slow cooker with butter or non-stick spray. Line the bottom with parchment paper if desired, then grease the paper.

In a large mixing bowl, whisk together the flour, baking soda, cinnamon, and salt. Set aside.

In another bowl or directly in the slow cooker (to minimize dishes), mash the bananas well. Stir in the melted butter, eggs, vanilla extract, granulated sugar, and brown sugar until thoroughly combined.

Gently fold the dry ingredients into the wet ingredients using a rubber spatula until barely combined. Do not overmix; a few streaks of flour are okay.

Pour the batter into the prepared slow cooker. Smooth the top lightly.

Cover and cook on HIGH for 2-3 hours or LOW for 4-6 hours, or until the center is set and a toothpick inserted comes out clean or with a few moist crumbs. Cooking time may vary depending on your crockpot, so check after 2 hours.

Turn off the crockpot, open the lid, and let the bread cool in the crock for 10-15 minutes. Then, carefully remove by lifting with the parchment paper and transfer to a wire rack to cool completely.