The level speed was reached at an altitude of 29,800 feet .
Also , by using a four-bladed propeller , rather than the three-bladed one used on the P-51A , the performance was greatly improved ; the XP-51B achieved a level speed of 441ï¿½mph at , over 100ï¿½mph faster than the Allison-engined P-51A at that altitude .
Eighth Air Force