When the P-51 airframe was paired with the Packard Merlin V-1650 engine instead of the Allison engine used on previous Mustangs, it was designated the P-51B or P-51C, and called the Mustang III by the RAF. The B and C Mustangs performed better at higher altitudes than earlier Mustangs, and drop tanks gave them an extended range that made them suitable as bomber escorts. The B versions were manufactured and North American Aviation's Inglewood, CA facility, while C versions were produced at North American's new plant in Dallas, TX.