Preheat oven to 375 degrees F.
Spray a 2-quart casserole dish with non-stick cooking spray and set aside. Bring a large pot of water to the boil. Salt water, add macaroni, and cook until al dente. Drain into a colander, run under cold water to stop the cooking, and then set aside.
While pasta is cooking, combine flour and milk in a large saucepan, whisking until blended. Cook over medium heat until thick, about 10 minutes, stirring constantly. Add cheeses and cook until melted and thickened. Remove from heat and stir in the macaroni and 1/4 teaspoon salt. Pour mixture into prepared casserole dish. Sprinkle bread crumbs over top. Bake until bubbly, about 30 to 40 minutes.