Combine chili powder, cumin, 1/2 tsp. salt, garlic powder, paprika, and ground pepper in a large bowl. Transfer 1 tsp. of the spice mixture to a medium bowl and set aside. Whisk 1 Tbsp. oil into the remaining spice mixture in the large bowl. Add chicken, sliced onion, and sliced red and green bell peppers; toss to coat.