Preheat the oven to 350 degrees F.
For the Dry Ingredients - In a medium size bowl, add flour, baking powder, baking soda, and salt. Whisk the ingredients together (whisking has a similar effect to sifting the dry ingredients). Set aside.
Cream the Butter, Sugar and Eggs - In a large bowl add butter and sugar. Beat together. Next, add the eggs and beat in with the butter and sugar mixture until creamy.
For the Batter - To the bowl with the butter mixture, add the dry ingredients a little at a time while mixing on low speed.
To the bowl, add mashed bananas. Using a large sturdy spoon, mix the bananas into the batter.
To the batter, add blueberries. Using a sturdy spatula, gently mix them into the batter.
Distribute the batter evenly into each loaf pan or paper liner.
Baking the Bread - Bake in a preheated oven until a wooden toothpick inserted into the bread comes out dry (about 45 minutes).
When the loaf pans are cool enough to handle, remove the loaves from each pan. Place them onto a cooling rack and let cool.