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Nicholas Kramer

Evolution of the Mustang

Brickworld Chicago, 2024


In 1940, the British Purchasing Commission made an agreement with the United States to supply additional aircraft for the Royal Air Force.  The RAF needed additional aircraft to supplement their own production as WWII carried on.  At the time, however, no US production aircraft actually met European requirements.  The Curtiss P-40 Tomahawk was the closest to meeting these standards, but Curtiss was already at production capacity.   The Purchasing Commission approached North American Aviation to produce the P-40 under license by Curtiss.  NAA replied that they could design and produce a newer, more modern aircraft using the same engine as the P-40, faster than establishing a production line for the older P-40 design.  At the time, NAA had not actually designed a fighter aircraft and the Commission insisted they study the P-40’s design and wind tunnel tests.  NAA purchased the plans from Curtiss and in May 1940, the new aircraft’s design plans were approved by the Commission and the Mustang project was signed with an initial order of 320 aircraft. 



The NAA prototype NA-73X rolled out in September 1940, after only 102 days, and flew about a month later.  The prototype used a new cooling system by moving it aft in the fuselage with the air traveling through a duct, rather than being positioned in the nose near the engine.  This reduced fuselage drag, and also had a slight thrust effect as the airflow exited the aircraft.  The fuselage was constructed entirely of aluminum to save weight.  The design was made to better accommodate mass production by having the aircraft manufactured in 3 main fuselage sections and 2 wing sections to be joined later on. The prototype was fitted with two .50 caliber AN/M2 Browning machine guns under the nose firing through the propellor arc, and an additional four .30 caliber M1919 Browning machine guns mounted in the wings, 2 per side. 



The “Mustang” Mk 1, as initially named by the British, used an in-line liquid-cooled V-12 engine, the Allison V-1710.  Superchargers, at the time, were typically reserved by the RAF for high altitude four-engine bombers.  Without a supercharger, most reciprocating engines suffer degraded performance with higher altitudes due to the lower air density and decreased oxygen levels.   The Allison engine put out 1,150 HP and used a Curtiss 3-bladed propellor.  While the Allison propelled the Mustang at about 390mph with a range of 750 miles, it was restricted to below 15,000 feet.  The Mk I was faster than a Spitfire and had double the range.  The Mk 1 served as a low level fighter, a ground attack aircraft, and as reconnaissance aircraft.  It entered service with the RAF in January 1942.  As the Mk I’s began being delivered to the British, two aircraft were provided to the US Army Air Corps for testing and evaluation, and named the XP-51.




In March of 1941, US Congress passed the Lend/Lease Act which allow the US to “lend” US aircraft to other countries, as long as it was “vital to the security of the United States.”  The US ordered 150 additional Mustangs to be sent to the RAF.  These new Mustangs eliminated the nose machine guns and changed the wing machine guns to four 20mm Hispano canons.  These aircraft were denoted as the Mustang Mk Ia.  


After the attack on Pearl Harbor, the US held back 57 Mustang Mk 1a’s from the order and outfitted them with .50 caliber machine guns instead of the 20mm canons.  Funding for the remainder of fiscal year 1942 had run out for fighter contracts, but funds did exist for dive bombers.  To keep the P-51 in production, a dive-bomber version was produced, and the A-36 Apache (called by some) was created.  Dive brakes were added to strengthened wings and bomb racks were added.  


RAF A-36, dive brakes extended.




US A-36, note chin machine guns.

The US Army’s second order of Mustang’s was the P-51A and 1,200 were ordered in August 1941.  These used a newer Allison engine, and had only the four .50 caliber machine guns in the wings.  They also had the ability to use drop tanks which greatly increased their range.  50 of these aircraft were given to the British to make up for the Mustang Ia aircraft that were held back.  The P-51A was quite similar to the A-36, except with the removal of the dive brakes and the removed nose guns.  The British named these aircraft the Mustang Mk II.  The newer engine used an improved supercharger, which increased its top speed to 412 mph at 10,000 ft. and made it the fastest mid-altitude fighter at the time.  A Rolls Royce Merlin 61 was suggested for the Mustang as it was shown to have high-altitude performance, but it would have required significant airframe modifications and resulted in significant production delays.  The Merlin 61 was never outfitted.






The British experimented with putting a Rolls Royce Merlin 65 engine (medium altitude) in the Mustang in 1942, similar to that used in the Spitfire Mk IX.  The Merlin 65 had a mechanical supercharger which gave the Mustang the high-altitude performance it needed.  It also used a four-bladed Rotol propellor also from a Spitfire Mk IX.  Five Merlin 65 equipped Mustangs were converted, and two were turned over to the United States for testing.



Following the testing and results of the new engine, NAA began incorporating them into production with the designation of Mustang B.  The United States’ Packard Motor company already had license to produce the Merlin engine, and in 1943, Packard Merlin P-51’s began being produced in NAA’s Inglewood, CA plant.  NAA supplemented the production with their new Dallas, TX plant, and these were denoted as the P-51C.  These Packard V-1650 Merlin engines were in-line V-12 liquid cooled engines.  They were much heavier than the Allison engines, so the airframe needed to be strengthened and the cowling redesigned as they were 5 inches taller.  The air intake was redesigned for the additional needed cooling, and a second duct was added for an oil cooler.  A larger Hamilton Standard 4-bladed propellor was used, which had hard-rubber cuffs at the base of the blades.  While the armament of the B was the same as the A, bomb racks were added which allowed the B to carry up to 500 lb bombs.  The new engine raised the service ceiling by 10,000 ft and gave it an additional 50mph of speed.  Later production B’s added an 85 gallon self-sealing fuel tank in the fuselage located behind the pilot.  While this increased the range, it pushed the center of gravity dangerously aft, and maneuvers were limited until the tank was under 25 gallons. 


By December 1943, Mustangs began flying long-range bomber escort missions over Germany and began outperforming the German Me 109’s and FW-190’s hindering the Allied bombers.  In just a few months, the Mustang’s superiority significantly reduced the heavy losses and damage to bombers during their missions.


Despite the improvements in altitude, speed, and range, the Mustang had some flaws that pilots complained about often.  The Mustang was prone to “snap-roll” in certain conditions at low speed.   Several crashes were the result of the horizontal stabilizer being completely torn off during maneuvers.   A strengthened stabilizer and a dorsal fin were needed to minimize the snap-roll.  Pilots also complained about the poor visibility to the rear, blocked by the interior of the razorback fuselage.  The canopy consisted of flat framed panels that could not be opened in flight and tall pilots had limited headroom.   A frameless moulded plexiglass canopy, called a Malcolm Hood, was retrofitted into some razorbacks, similar to that used in Corsairs.  These new canopies were sliding, rather than using folding panels.


P-51B, with Malcolm Hood




Combat experience led to many design changes in the Mustang.  New moulding techniques used for streamlining bomber noses allowed for the introduction of a teardrop shaped bubble canopy.  This single piece of moulded plexiglass gave a full 360 degree view for the pilot.  The windscreen was also redesigned and was “bullet-resistant.”  The undercarriage was redesigned which necessitated an enlarged filet to the leading edge.  The gear was canted further forward which lowered the nose to allow the pilots a slightly better view when operating on the ground.  Navigation lights were moved to the wing tips and a retractable landing light was used in the wheel well, rather than a fixed light on the leading edge of a wing.  Armament was increased to six AN/M2 Browning .50 caliber machine guns.  The six guns were now mounted vertically along the wing's dihedral, rather than canted inboard and positioned parallel to the ground.  The canted mount caused frequent jamming.  The wing racks were strengthened and could now carry up to 1,000 lb ordnance, although only 500 lbs was recommended.  Some D's were outfitted with rails to allow up to ten 5” HVAR rockets.  The British named the D variant the Mk IV.  The P-51D was the most produced version of the Mustang, with some 8,100 built.








The P-51D’s that were produced at the Dallas, TX plant were outfitted with an Aeroproducts propellor which was slightly smaller in diameter and was hollow.  These propellors proved unreliable and they were eventually replaced with Hamilton Standard propellors.  The British named the K variant the Mustang Mk IVa.


After a series of experimental aircraft variants, the H variant was the final production version.  The fuselage was lightened by 600 lbs and two of the Brownings were removed.  A new Merlin engine was used, increasing the engine output to over 2,200 HP.  The fuselage was strengthened, despite being lighter, and the tailfin was lengthened.  The lighter weight and more powerful engine led to the Mustang H as being one of the fastest production piston-engined aircraft to see service.  Over 2,000 were ordered in preparation for the invasion of Japan, but the war ended with only 555 built, with the rest cancelled.  None of the H models saw combat in either WWII or Korea.


P-51D, Top.    P-51H, bottom.


An L variant was planned which was similar to the H, but used a newer, even more powerful engine - putting out 2,270 HP.  The L variant would have been produced at the Inglewood plant.  The Dallas plant's version, the M variant, used a different engine that lacked the liquid cooling, which would have limited its power output to a decreased amount.  Only 1 was built, which was the last production Mustang ever made, in 1945.


In 1947, all P-51’s in the USAF were redesignated as the F-51.

A total of 15,586 Mustangs were built by North American Aviation.  Commonwealth Aviation Corporation in Australia produced another 200, and Cavalier Production produced 19.  Today, only about 170 are still airworthy, with additional aircraft on display or undergoing restoration.





Empty Weight:

Gross Weight:

Max Takeoff Weight:

Fuel Capacity:


32' 3"

37' 0"

13' 4.5" 

7,635 lbs.

9,200 lbs

12,100 lbs.

269 US Gal.

Packard V-1650-7 V-12

     1,490 hp at 3,000 rpm

     1,720 hp at WEP

Max Speed:

Cruise Speed:

Stall Speed:


Service Ceiling:

Rate of Climb:

440 mph

362 mph

100 mph

1,650 miles (with ext. tanks)

41,900 feet

3,200 ft/min







Six  AN/M2 Browning .50 caliber machine guns

     Rate of Fire: 600-800 rds/min (each gun)

     Number of Rounds:  1,840 Total  (380 rds for each inboard, 270 rds for both outer

6 or 10 - 5" T-64 HVAR rocket

100lb, 250lb, or 500lb bombs

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All information and photos was taken from various internet sources.  I do not own nor claim credit for any of the photos taken.

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