In large microwavable bowl, place cheese cubes, butter and milk. Microwave uncovered on High 8 to 10 minutes. Stir with whisk; microwave on High 4 minutes longer. Stir again; repeat microwaving until all cheese is melted and mixture can be stirred smooth, a total of about 15 minutes.
Heat oven to 350°F. Cook macaroni in boiling salted water 3 minutes less than directed on box (macaroni should be firm). Drain; place in 15x10-inch (4-quart) glass baking dish. Pour melted cheese sauce over macaroni; mix well. Stir in shredded cheese, tomatoes and spinach.
Cover dish with foil; bake 30 minutes. Uncover; sprinkle bacon and bread crumbs evenly over top. Bake 5 minutes longer or until top is lightly browned. Cool at least 10 minutes before serving.