Place the chicken breasts in the bottom of the crockpot.
Add the sliced bell pepper and onion on top of the chicken.
Pour the can of diced tomatoes over the vegetables and chicken.
Sprinkle the fajita seasoning mix evenly over the top.
Drizzle the olive oil over the mixture, and season with salt and pepper to taste.
Cover the crockpot and cook on low for 6-8 hours or on high for 3-4 hours, until the chicken is cooked through and tender.
Once cooked, shred the chicken using two forks and stir to combine with the vegetables and juices.
Serve the chicken fajita mixture in warm flour tortillas and add your favorite toppings.