Heat 2 tablespoons olive oil in a large skillet over medium high heat. Sprinkle chicken with salt and pepper. Add chicken to skillet and toss just until cooked through, about 5 minutes. Using slotted spoon, transfer chicken to bowl. Add onions, bell pepper, and garlic to skillet. Sprinkle with salt and pepper and saute until beginning to soften, about 4 minutes. Add chili powder, oregano, and chipotle powder; stir 2 minutes. Add tomato paste; stir 1 minute. Add crushed tomatoes and broth; bring to a boil. Reduce heat to medium and simmer sauce until slightly thickened and flavors blend, stirring occasionally, about 15 minutes.
Coarsely shred chicken and add to sauce. Simmer until heated through, about 3 minutes. Season filling to taste with salt and pepper. (Can be made 2 days ahead. Chill uncovered until cold. Cover and keep chilled. Rewarm before continuing.).
Preheat oven to 450 degrees. Stir cilantro into filling. Place 1 tortilla on work surface. Sprinkle with 1/2 cup grated cheese down center of tortilla, leaving 2 inch plain border at top and bottom. Sprinkle about 1/4 cup chopped tomatoes over, then spoon 3/4 cup chicken filling atop cheese and tomatoes. Sprinkle with generous 2 tablespoons lettuce. Fold top and bottom plain borders of tortilla over filling, then fold 1 side over and roll burrito over to enclose filling. Transfer, seam side down, to 15x10x2 inch baking dish. Repeat with remaining tortillas and filling.
Spoon 1/4 cup enchilada sauce over each burrito. Sprinkle remaining 1 1/2 cups cheese over burritos. Bake uncovered until heated through and cheese melts, about 15 minutes.
To make the enchilada sauce: Heat 3 tablespoons olive oil in a heavy large saucepan over medium-high heat. Add chili powder, garlic, and oregano; stir 1 minute. Stir in crushed tomatoes, chicken broth, and tomato paste. Bring to boil; reduce heat to medium-low and simmer until flavors blend and sauce thickens slightly, stirring occasionally, about 15 minutes. Season to taste with salt and pepper. Can be made 2 days ahead. Refrigerate uncovered until cold, then cover and keep refrigerated.