Heat a skillet over medium heat. Add the ground beef and cook until browned, breaking it up with a spoon. Drain any excess fat.
Add the diced onion, bell peppers, and garlic to the skillet. Cook for 3-4 minutes until softened.
Add the browned meat and vegetables to the slow cooker.
Stir in the diced tomatoes, tomato sauce, kidney beans, black beans, broth, tomato paste, chili powder, cumin, smoked paprika, garlic powder, cayenne pepper (if using), salt, and pepper.
Cover the slow cooker and cook on low for 6-8 hours or high for 3-4 hours. Stir occasionally if possible.
About 30 minutes before serving, taste the chili and adjust the seasoning as needed. Add more chili powder or cayenne if you like it spicier.
Ladle the chili into bowls and add your favorite toppings. Enjoy with cornbread, tortilla chips, or rice.