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



Later modifications

As production carried on (800 more Mustangs of the next block P-51B-5-NAs were built), more bugs were ironed out and more modifications were made to the P-51:

  • One key element was the ongoing search for longer range. Even with drop tanks, the Mustang still did not have enough range to escort the bombers all the way to Berlin and back.
    There was some empty space in the rear fuselage, between the pilot's seat and the radio equipment and so NAA engineers devised an 85 gallon (322 liter) self-sealing fuel tank to fit in between. This brought the total fuel capacity to 419 US gallons (including 2 drop tanks) and extended the range to 2200 miles which was more than the distance from England to Berlin and back.
    The addition of the extra fuel tank started with the P-51B-7NA (P-51B-5-NAs where the tank was added were renamed to -7NA) and the P-51C-3NT versions (renamed from the -1 line).
    This gave the Mustang the extra range it so desperately needed but caused some major other problems. The extra weight, so far to the back of the airframe, moved the center of gravity aft and made directional stability troublesome until most of the fuel was consumed.
    The British also made their own contribution to the range problem in the form of a new drop tank that accommodated 409 liters (108 US gallons) and which was made of plastic-impregnated paper. This sounds a little crazy, particularly since the fuel would rot the tank if left in it for more than eight hours. Nonetheless, the paper drop tanks were perfectly effective, and were lighter and cheaper than metal drop tanks. Dropping them over German territory also did not provide the enemy with aluminum they could scavenge for their own war effort. Production rates of the paper drop tanks eventually reached 24,000 a month.

    With this full fuel load, getting the P-51 off the ground was dangerous to say the least. Because of the added weight to the back of the aircraft, getting the tail wheel to lift on takeoff was nearly impossible so pilots just lifted the main wheels off the ground first and waited for enough airspeed to lift the entire plane off the ground.
    Also, the fighter groups didn't have the luxury of waiting for one aircraft to clear the runway before sending another out. The result was sometimes a hideous fireball on the runway, followed by an even more disastrous pile-up that incinerated men and machines.
    P-51B/C fuselage tank
    Left: P-51B/C without center fuel tank - Right: P-51B/C with center fuel tank installed

    Normal procedure was to burn the fuel in the drop tanks first, but obviously battle with the center fuel tank loaded could easily throw the Mustang out of control. Procedure was altered to burn the fuel in the center tank first, but this would take the Mustang well over enemy territory and they were often forced to drop full fuel tanks before engaging the Luftwaffe.

    Maneuvers proved restricted until the center tank was at 25 gallons. This resulted in the pilots carefully exercising their fuel management in the first hour or so after takeoff, before they entered combat. They soon learned to burn most of the fuel in the center tank first, then switched over to the drop tanks before they had to drop them and fight, then back to the center tank and finally to the main fuel tanks in the wings.

    Nonetheless, there was a war on and something was needed right away, so the third tank became standard equipment in all future versions.

    The added stress in maneuvering caused by the tail heavy aircraft with the additional center tank also lead to the replacement of the fabric covered elevators to metal covered elevators.

    In practice, the total amount of fuel put in the center tank was usually limited to 65 gallons to reduce these problems.
  • In later production, the engine was replaced by the more powerful V-1650-7 Merlin, resulting in 1,450 horsepower (50 more than the -3 version). The new Merlin also had a war emergence power (WEP) rating of 1695 horsepower at 10,300 feet. WEP means that the engine is limited from redlining with a safety wire, but in the case of an emergency the pilot could push the throttle past the stop, thus breaking the wire and providing a boost in power for a short period of time. The engines could be redlined for about five minutes without resulting in any damage. The maximum speed was reduced from 440 mph to 435 mph at 20,000 feet but increased from 430 mph to 439 mph at 25,000 feet.
    The newer Merlin was introduced at the end of the P-51B-10NA block and in the P-51C-5NT block
  • Alternate filtered air intakes were added on each side of the chin scoop for use in dusty conditions. These were added early on in the first block of P-51Cs (-5 block).
    P-51 filtered air intake
  • Mustang gunsThe canted installation of the four 0.50 caliber Browning machine guns had a fault which was not discovered until they were used in combat. The ammunition belt feed mechanism, designed to lift 35 pounds of ammunition, was not enough to pull the ammunition through the articulated ammo track during high-G maneuvers. This resulted in frequent gun jams.

    A field modification was made in the B/C-models which consisted in the installation of booster motors with star wheels salvaged from Martin B-26 turrets. Those provided the extra energy to properly feed the guns under high stress maneuvers.

    The gun problem would be definitely fixed on later D-models which had a different armament layout.

    Note: The ammunition first used in combat was the traditional loading of 2 armor piercing (black tipped), 2 incendiary (blue tipped) and one tracer (red tipped) with the pattern repeated throughout the belt. It was later discovered that this stream of tracers was of as much value to the enemy as our pilots, so the tracers were removed and only used near the end of the belt to warn the pilot that his ammo was almost exhausted. About March or April 1944 the M8 silver-tipped armor-piercing incendiary projectile began to flow through the supply system and was originally only issued to "management"--flight leaders or better because of its scarcity.
  • A particularly alarming problem, to say the least, was the occasional tendency of the wings to fall off during a high-speed dive. Ironically, Thomas Hitchcock, who had done much to promote the Merlin Mustang, was killed over England in a P-51B in this way in April 1944. These structural failures proved to be due to the tendency of wing ammunition doors or the main landing gear to open under high-G conditions, producing excessive forces on the wing. These problems were eventually resolved.

  • Another complaint in the P-51s was the poor all-round visibility from the cockpit. Also, the cockpit could not be opened in normal flight (only jettisoned in case of emergency) and it was difficult to get out of. The RAF came up with a solution in which they replaced the 3-piece canopy panels with a single-piece bubble canopy. This was called the “Malcolm Hood” (named after the British company who designed it). The new hood was opened by sliding it back on rails in one piece, which made getting in and out a lot easier. It could also be opened in flight.
    The “Malcolm Hood” was refitted in the field to most of the RAF Mustang IIIs and to some of the USAAF P-51s.
    Courtesy of In Action 45 - P-51 Mustang by Squadron/Signal Publications
    P-51 with standard canopy
    Above: P-51C-10NT Princess Elizabeth with the standard canopy - Below: P-51B-5NA Hell-Er-Bust with the Malcolm hood canopy installed
    P-51 with Malcolm hood
  • The problems with misidentification with the Messerschmitt Bf-109 continued. White stripes were painted on the nose, wings, and tail, but with only limited effectiveness. On February 10th, 1944, for example, Glenn Eagleston was shot up by a P-47 over Germany and only barely managed to make it back to England to bail out.

    Before the year was out, the problem would be reduced by the simple measure of delivering Mustangs to England in natural metal finish. The USAAF wanted to take on the Luftwaffe, not hide from them. Eliminating the paint actually reduced aircraft weight and slightly improved performance. RAF Mustangs would continue to be painted in camouflage until near the end of the war.

Despite these mods, the Mustang experienced low speed handling problems that could lead to an involuntary snap-roll under certain conditions. Several crash reports tell of crashes because the horizontal stabilizers were torn off during maneuvering. As a result, a mod kit consisting of a dorsal fin was manufactured. These kits became available in August 1944. One report stated:

"Unless a dorsal fin is installed on the P-51B, P-51C and P-51D airplanes, a snap roll may result when attempting a slow roll. The horizontal stabilizer will not withstand the effects of a snap roll. To prevent recurrence the stabilizer should be reinforced in accordance with T.O. 01-60J-18 dated 8 April 1944 and a dorsal fin should be installed. Dorsal fin kits are being made available to overseas activities"

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