Preheat oven to 375°F. Generously mist a 9-by-13-inch baking pan with cooking spray.
Warm oil in a skillet over medium-high heat. Sauté red and green bell peppers until softened, about 4 minutes. Stir in green chiles, remove from heat and cool slightly.
Whisk flour, cornmeal, baking powder and salt in a bowl. Whisk milk, eggs and butter in a separate bowl until smooth. Stir milk mixture into flour mixture to blend. Stir in bell pepper mixture and jalapeño. Do not overmix. Pour batter into pan, smooth top and bake until edges are brown and a tester inserted in the center comes out clean, 30 to 35 minutes. Let cool. Serve with butter.