P-51 B/C (Mustang III)

Enter the Merlin

The X-factor

Design changes

Production expansion: is it a B or a C?

Later modifications

Operational service

Onto the legend


Serial numbers

3-Way drawings



Design changes

The Merlin powered Mustangs incorporated the following changes:

  • A Packard Merlin-65 engine
  • A Hamilton Standard four-bladed 11ft 2in (3,4m) propeller to turn the extra power of the Merlin into thrust in the thin air of higher altitudes
  • Stronger underwing racks to accommodate two 1,000 pound bombs or the same weight in drop tanks
  • A strengthened airframe because of increase in power
  • New ailerons
  • A slightly fatter nose. The cowling was 5 inches taller (130mm) than its predecessors
    Courtesy of In Action 45 - P-51 Mustang by Squadron/Signal Publications
  • A Relocated carburetor intake, from above to below the nose: the two-stage Merlin had 2 superchargers mounted in series. This meant that the rapid compression of air passing through the 2 superchargers resulted in a considerable increase in air temperature (+200 degrees centigrade). As a result, an intercooler was added but this required an extra radiator. NAA engineers moved the carburetor air intake from above the nose to below the nose in order to accommodate the Merlin's updraft induction system
    P-51B Carburator Air Induction system
  • The intercooler radiator itself was added to the radiator group already located inside the scoop underneath the fuselage. As a result, the housing underneath the fuselage was made considerably deeper and had a sharp-angled inlet. There was now a 2-inch space between the inlet and the fuselage to stay clear of the turbulent airflow underneath the wing.
    The oil cooler was relocated to the front of the scoop and provided with its own ventral exit door.
    The whole coolant housing also had an enlarged duct and bigger exit door
    Courtesy of In Action 45 - P-51 Mustang by Squadron/Signal Publications
  • Standard armament was four 0.50 caliber Browning M2/AN machine guns in the wings. The outer guns had 280 rounds of ammunition each and the inner guns 350 rounds. The nose mounted guns were removed due to the bigger engine
  • N-3B Gunsight
    N-3B gunsight

Back in Britain, the two final Mustang Xs would be flown in early 1943 and evaluated by the USAAF, but by that time they were of no critical importance. Even before the first Merlin Mustangs flew, back in August of 1942, the USAAF had ordered 400 P-51Bs, gambling that they would prove everything that was hoped for them.

Rolls-Royce wanted to convert 500 RAF Allison Mustangs to Merlin power, but the resources were simply not available. The Americans were not under such constraints, and so in early 1943 the RAF ordered 1,000 P-51Bs with the designation of "Mustang III".


Production expansion: is it a B or a C?

The USAAF was now finally sold for the Mustang as it was now a fighter which was able to compete with the Luftwaffe's FW-190 and later models of the Bf-109. They placed an order of 2,200 P-51B's.

NAA was now able to produce one of the hottest fighters at the time but was unable to manufacture the P-51 in such large quantities. To be able to keep up production with the orders they moved the B-25 program to Kansas City , thus dedicating the Inglewood plant in California to the Mustang. They also expanded their Fort Worth ( Dallas , Texas ) plant to start up a second line of P-51s. Until that time, the Dallas plant was solely used to manufacture trainers.

Inglewood Mustangs were designated P-51Bs and the Dallas built Mustangs were designated as P-51Cs. Other than this the P-51B and P-51C are exactly identical. The production plant can also be deducted from the aircraft's designation: Inglewood Mustangs have a -NA suffix and Forth Worth Mustangs a -NT suffix (the following text about the P-51B also applies to the P-51C unless otherwise noted). The NAA model numbers were NA-102 for the P-51B, NA-103 for the P-51Cs on 1942-1943 budgets and NA-111 for the P-51Cs in the 1944 budget.

P-51C Cutaway

The first P-51Bs rolled out of the Inglewood plant in May 1943 and the first P-51B flight was on May 5th, 1943. The first P-51C took to the skies three months later on August 5th, 1943.

There were 400 P-51B-1-NAs and 250 P-51C-1-NTs built. They all had the Packard V-1560-3 (or Merlin 61) engine. 398 P-51B-10-NAs, 390 P-51B-15-NAs, and 1,350 P-51C-10-NTs were built, all powered by the V-1650-7 engine.

71 P-51B-10-NAs and 20 P-51C-10-NTs were fitted with two K24 cameras in the rear fuselage and were redesignated as F-6C-NA or NT reconnaissance aircraft.

The USAAF would put down an additional order of 1,350 P-51s and in the end 1,990 -B models and 1,750 C models were manufactured . In total, Inglewood built 1,988 P-51Bs and Dallas built 1,750 P-51Cs.

P-51B/C cockpit details
Above: P-51B/C cockpit detail - Below: The new Mustang had beautifull clean lines and resulting low drag which contributed greatly to its amazing performance
P-51C-10NT Princess Elizabeth at Duxford

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