In a large skillet, cook the ground beef, red bell pepper, and onion over medium heat until the beef is thoroughly cooked and the vegetables are tender, about 8-10 minutes. Make sure to crumble the beef as it cooks. Drain any excess fat.

Add the drained diced tomatoes, corn, tomato sauce, chili powder, and salt to the skillet. Stir to combine and bring the mixture to a boil.

Reduce the heat and stir in the cooked brown rice. Continue to cook until the entire mixture is heated through.

Fill the warmed taco shells with the beef and rice mixture. If desired, top with shredded lettuce, chopped fresh tomatoes, and a dollop of reduced-fat sour cream.