Preheat your oven to 425°F (220°C).
In a large mixing bowl, combine the olive oil, chili powder, cumin, garlic powder, onion powder, paprika, salt, and black pepper to create a marinade.
Cut the flank steak into thin strips and toss it in the marinade, ensuring all pieces are well-coated. Let it sit for about 10 minutes.
On a large sheet pan, arrange the marinated steak, sliced bell peppers, and sliced onion evenly.
Roast in the preheated oven for 20-25 minutes, stirring halfway through, until the steak is cooked to your desired doneness and the vegetables are tender.
While the fajitas are roasting, warm the flour tortillas in a dry skillet or wrap them in aluminum foil and place them in the oven for the last 5 minutes.
Remove the sheet pan from the oven and let it rest for a few minutes.
Serve the steak and vegetables in the warm tortillas and add your favorite toppings.