Place noodles and red pepper in a medium-size bowl and pour boiling water over top. Let noodle mixture sit for 5 minutes, stirring, occasionally.
Whisk together peanut butter, soy sauce, vinegar, sugar and red pepper flakes in a small bowl.
Drain noodles and peppers, setting aside 1/2 cup water. Place noodle mixture in a serving bowl and add chicken.
Pour peanut sauce over noodles and toss to combine, adding cooking water as needed to thin sauce. Sprinkle with scallions and serve.