In a slow cooker, combine the sliced flank steak, red and green bell peppers, onion, and garlic.
In a small bowl, mix together the chili powder, cumin, smoked paprika, salt, black pepper, and cayenne pepper. Sprinkle the spice mixture over the beef and vegetables in the slow cooker.
Pour the beef broth and lime juice over the top. Stir gently to combine all ingredients.
Cover and cook on low for 6-8 hours or on high for 3-4 hours, until the beef is tender.
Once cooked, shred the beef with two forks and mix it back into the vegetables and juices.
Warm the tortillas in a skillet or microwave.
Serve the beef fajitas in tortillas with your choice of toppings such as sour cream, guacamole, salsa, shredded cheese, and fresh cilantro.