Fill a saucepan with 2 inches of water. Place an unopened can of sweetened condensed milk into the water. Bring to a simmer and let cook for 3 hours. Monitor the water closely, to make sure there is always water in the pan. Remove can from heat and let cool for 10 to 15 minutes.
Carefully open can and pour contents into pie crust. Slice bananas over the top and cool in the refrigerator. Before serving, spread with whipped topping.