In a slow cooker, combine the sliced flank steak, red and green bell peppers, onion, and minced 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 steak 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 steak is tender.
Once cooked, shred the steak with two forks and mix it with the vegetables.
Warm the flour tortillas in a skillet or microwave.
Serve the steak fajitas in tortillas, garnished with fresh cilantro and lime wedges.