Cook beef over medium heat until done; drain. Meanwhile saute onion and bell pepper in oil until tender. Add onion and pepper to beef. Stir in beans, tomatoes, chili powder, salt, cumin, pepper and garlic powder. In a slow cooker place two tortillas side by side, overlapping if necessary. Layer with a forth of the beef mixture and a fourth of the cheese. Repeat layers three more times. Pour enchilada sauce over all. Cover and cook on low for 2 to 2-1/2 hours or until heated through.