Coat the bowl of a slow cooker with cooking spray. Place onion, garlic, green pepper, red pepper, and ground beef in the bottom of the slow cooker.
In a medium bowl, combine tomatoes, chili powder, tomato paste, cumin, oregano, salt, and chipotle peppers. Pour over ground beef.
Cover and cook on high for 6 hours or low for 8 hours. Stir in beans during the last 30 minutes of cooking time.
Serve with crushed tortilla chips and sour cream, if desired.