The P-51 was designed (as the NA-73) in 1940 at Britain's
request.
In April 1938, the
British government established a purchasing commission in
the United States, to organize the manufacturing and supply
of American fighter aircraft for the RAF. At the time, the
choice was very limited as no U.S. aircraft already flying
met European standards, with only the Curtiss P-40 Tomahawk
coming close. The Curtiss-Wright plant was running at
capacity, so even P-40s were in short supply.
North
American Aviation (NAA) approached the RAF to sell a new
medium bomber, the B-25 Mitchell. Instead, RAF asked if NAA
could manufacture the P-40 Tomahawk under license from
Curtiss. North American felt that they could produce a
better aircraft with the same engine in the air in less time
than it would take to set up a production line for the P-40.
The Commission stipulated armament of four .303 in (7.7 mm)
machine guns, the Allison V-1710 liquid-cooled engine, a
unit cost of no more than $40,000, and delivery of the first
production aircraft by January 1941, in March 1940, 320
aircraft were ordered.
The
prototype NA-73X was rolled out in early August and first
flew on 26 October 1940 respectively just 117 and 178 days
after the order had been placed. The
design showed such promise that in 7 July 1941 the
Army
Air Forces placed an
order for 150
P-51s. After the attack on Pearl Harbor priority had to be
given to building as many of the existing fighters - P-38s,
P-39s and P-40s. The 150 NA-91s were designated
P-51 by the
newly formed USAAF and were initially named Apache, although
this was soon dropped, and the RAF name, Mustang, adopted
instead. The USAAF did not like the mixed armament of the
British Mustang Is and instead adopted an armament of four
long-barreled 20 mm (.79 in) Hispano Mk II cannon, and
deleted the .50 cal engine cowling mounted weapons. The
British designated this model as
Mustang Mk IA.
A number of aircraft from this lot were fitted out by the
USAAF as F-6A photo-reconnaissance aircraft. The British would fit a number
of Mustang Mk Is with similar equipment.
The first version of the P-51 to see operation service in the
US was the A-36 Apache dive-bomber that first flew in 1942.
About 500 of these "near-Mustangs" were built and saw action
in the Italian campaign and in India.
The US used the
A-36 in the Mediterranean theatre with first deliveries
beginning in early 1943. When not in the ground attack role,
the A-36 was essentially a low-altitude P-51A and was used
as a fighter. The A-36 scored 101 air-air victories during
WWII. The A-36 proved to be a very stable platform for
accurate weapons delivery, 177 were lost in action mostly
due to the dangerous mission of low-level operations.
The
next versions to see service was the P-51 B /C built in
Inglewood, CA and
Dallas, TX which was to manufacture
P-51Cs. The P-51B and P-51C were essentially identical, with
the only distinguishing feature being the serial numbers.
The engines were supplied by the Packard Company, which had
reached an agreement with Rolls Royce to license produce the
Merlin under the American designation V-1650.
The NAA P-51B/C represented a quantum leap in high altitude
performance and range, allowing them to escort the heavy
bombers all the way to their targets deep into enemy
territory.
The first P-51B
exited the assembly line and flew in May of 1943 with the
P-51C model flying 3 months later. In search of even more
range, an 85-gallon fuselage tank was installed behind the
pilot's seat. This began with the P-51B-7NA (taken from -5
lines) and the P-51C-3NT versions (taken from the -1 line).
With this added range came a price of directional stability.
Problems for the P-51B and C models included poor visibility
towards the rear and gun jamming during high G maneuvers.
The RAF found the best solution (at the time) for the
visibility issue was installing a "Malcolm Hood" which was a
bubble type canopy around the pilot's area of the birdcage.
The new hood was on rails sliding rearward for an easy in
and out. The old canopy was hinged and more difficult for
egress. The "Razorback" Merlin Mustangs P-51B and P-51C
remained in service until the end of the war.
P-51Bs and P-51Cs started to arrive in England in August and
October 1943. The P-51B/P-51C versions were sent to 15
fighter groups that were part of the 8th and 9th Air Forces
in England and the 12th and 15th in Italy (the southern part
of Italy was under Allied control by late 1943). Other
deployments included the China Burma India Theater (CBI).
Allied strategists quickly exploited the long-range fighter
as a bomber escort. It was largely due to the P-51 that
daylight bombing raids deep into German territory became
possible without prohibitive bomber losses in late 1943. A
number of the P-51B and P-51C aircraft were fitted for photo
reconnaissance and designated
F-6C.
The
shortcomings of the B/C models were well known and evolved
into a new model of P-51, the P-51D. The new line,
designated NA-109, P-51D, was started after the USAAF
ordered 2,500 in July 1943. Interesting is that the XP-51D
did not have a test flight until November 17, 1943, well
after the first order was placed. Deliveries to fighter
units began March of 1944 and a good supply was on hand for
the Normandy Invasion, or D-Day.
The signature change in the P-51D line was the new bubble
canopy. The U.S. was behind the British in canopy
development. A British company had designed and built the
"Malcolm Hood" which improved visibility to the rear of the
P-51B/C models. The U.S. was not unaware of the advantages
of a bubble canopy design. NAA had built a wooden model of
the P-51 with a bubble canopy for wind tunnel testing. The
technology to build large curves of Plexiglas "distortion
free" at that time was being invented and developed. The
cut-down back for the new bubble canopy lead to a loss of
surface area. This caused the P-51D to have directional
problems (especially with full fuel in the fuselage tank
creating an aft CG) for all but the most proficient pilots.
The solution was to add a fillet (often called the "dorsal
fin") to the vertical stabilizer that extended down and
towards the front. The fillet was also added to other P-51
models already in the field.
The addition of the 85 US gallon (322 l)
fuselage fuel tank, coupled with the reduction in area of
the new rear fuselage, exacerbated the handling problems
already experienced with the B/C series when fitted with the
tank, and led to a fillet being added to the base of the
vertical tailfin. The new model Mustang also had a
redesigned wing; alterations to the undercarriage up-locks
and inner-door retracting mechanisms meant that there was an
additional fillet added forward of each of the wheel bays,
increasing the wing area and creating a distinctive "kink"
to the leading edges of the inner wings.
The armament was
increased with the addition of two more .50 in (12.7 mm) M2
Browning machine guns, bringing the total to six. The inner
pair of machine guns had 400 rpg, and the others had 270 rpg,
for a total of 1,880.
The wing racks fitted to the P-51D/P-51K series were
strengthened and were able to carry up to 1,000 lb (450 kg)
of ordnance, although 500 lb (230 kg) bombs were the
recommended maximum load.
Later models had removable under-wing 'Zero Rail' rocket
pylons added to carry up to ten T64 5.0 in (127 mm) H.V.A.R
rockets per plane. The gunsight was changed from the N-3B to
the N-9 before the introduction in September 1944 of the
K-14 or K-14A
The P-51D became the most widely
produced variant of the Mustang. During 1945–48, P-51Ds were
also built under license in Australia by the Commonwealth
Aircraft Corporation. A Dallas-built version of the P-51D,
designated the P-51K, was equipped with an 11 ft (3.4 m)
diameter Aeroproducts propeller in place of the 11.2 ft
(3.4 m) Hamilton Standard propeller. The hollow-bladed
Aeroproducts propeller was unreliable, due to manufacturing
problems, with dangerous vibrations at full throttle and was
eventually replaced by the Hamilton Standard. By the time of
the Korean War, most F-51s were equipped with "uncuffed"
Hamilton Standard propellers with wider, blunt-tipped
blades.
The photo reconnaissance versions of the P-51D and
P-51K were designated
F-6D and
F-6K respectively. The RAF assigned the name
Mustang Mk IV
to the P-51D model and
Mustang Mk IVA
to P-51K models. The P-51D/P-51K started arriving in Europe
in mid-1944 and quickly became the primary USAAF fighter in
the theater. It was produced in larger numbers than any
other Mustang variant. Nevertheless, by the end of the war,
roughly half of all operational Mustangs were still P-51B or
P-51C models.
During May
1944, the 317th, 318th and 319th Fighter Squadrons of the
325th Fighter Group nicknamed "The Checkertail Clan"
transitioned out of the rugged P-47 Thunderbolt into the
Mustang.
With their new Mustangs, the squadron was able to fly
missions farther into enemy territory, and to escort bombers
from a pre-target rendezvous point, over the target, and
well along the course back to base. This meant that one
group flying P-51s would be able to supply the cover that
had previously required two or three groups flying P-47s.
On 7 May 1945, hours before the end of the war in Europe the
325th flew its last combat sortie, an mission escorting
Halifax bombers of the Royal Air Force's 148 Squadron, when
the propellers of the five P-51's came to a halt, the war
ended for the Checkertail Clan. The final record for the
325th in the P-51 was 342 missions, 10,596 sorties flying
45,370 combat hours with 246 aerial victories vs. 75 losses.
At war's end, 5,541 Mustangs were on hand. Surplus machines
were sold or given to Australia, China, Cuba, Denmark, the
Dominican Republic, France, Indonesia, Israel, Sweden,
Korea, Switzerland, Italy and Canada were the Mustangs
served in Reserve Air Defense units . Many were also passed
on to the USAF Air National Guard units, locally flown by
the WA ANG's 116th Fighter Squadron (1947-1950 & 1952-1954)
at Felts Field near Spokane WA. Other Squadrons in the
region, Oregon's 123d Fighter Squadron (1946-1951 &
1952-1953) Montana's 186th Fighter Squadron (1946-1953) and
the 190th Fighter Squadron (1946-1953) were assigned the
Mustang at their home base and other bases during their
activation during the Korean War in the early 50's.
During the Korean War, F-51Ds (as they were redesignated in
1948) were used by active duty units primarily for close
support of ground forces until the type was withdrawn from
combat in 1953. Mustangs have been popular for many years on the unlimited
racing circuit. In 1984, the Piper Enforcer, a turboprop
design based on the P-51, was marketed to smaller air forces
as a light attack aircraft although none was bought.