Heat oil in a large skillet. Add onion, carrots and green pepper and cook until tender.
Add tomatoes and chili powder. Reduce heat, cover and simmer on low 5 minutes. Uncover and simmer 5 minutes more, stirring occasionally.
Stir in beans and cook until heated through, about 5 minutes.
Fill each pita with approximately 1/3 cup bean mixture just before serving.