Preheat oven to 350 degrees F. Grease two 9 x 13-inch pans.
Combine the flour, baking powder, baking soda, salt, and cinnamon and set aside. In a separate bowl, mix the pumpkin, vegetable oil, eggs, and sugar. Add the flour mixture and stir just until combined. Mix in the walnuts.
Spread the batter into the prepared pan and bake in the preheated oven until a toothpick inserted in the center comes out clean, about 25 minutes. Cool on wire rack and frost bars with Cream Cheese Frosting.