Preheat oven to 300 degrees Fahrenheit. Lightly grease 2 (9x5-inch) bread loaf pans with nonstick cooking spray.
In a large mixing bowl whisk together 1 cup unsweetened applesauce, 3 large eggs, and 1 teaspoon vanilla extract until combined. Whisk in 1 cup granulated sugar and 1 cup brown sugar until smooth.
Stir in 3 cups all-purpose flour, 1 teaspoon baking soda, 1 teaspoon salt, and 2 teaspoons ground cinnamon until thoroughly mixed in. Then use a rubber spatula to fold in 3 cups diced granny smith apples.
Divide batter equally into the two prepared pans. Bake in the 300-degree oven for 90 minutes, until a toothpick inserted into the center comes out clean.
Remove pans from oven and let cool for 10 to 15 minutes on a wire rack before removing the loaf from the pan. Cool completely on a wire cooling rack.