One problem with the P-51B/C was the poor view from the cockpit, particularly rearward. The Malcolm hood tried on the P-51B/C was an improvement, but a better solution was sought. In Jan 1943, while in England, Wright Field's Col Mark Bradley saw how the new "bubble" or "teardrop" canopy had given Spitfire and Typhoon pilots an unobstructed 360° view. Back in the USA in June, he began exploring the possibility of such canopies on AAF planes. Republic put a bubble canopy on the P-47D in record time and Bradley flew it to Inglewood to show to Kindelberger. After discussions with the British and examination of their clear-blown "teardrop" canopies, NAA cut an agreement with the Army to test a similar canopy on a Mustang.

    P-51B [43-12101], redesignated XP-51D, was selected to be modified to test the new canopy that provided a 360° view with virtually no distortion. The large rear section did not reach its point of maximum height until a point well aft of the pilot's head—wind tunnel tests showed that shape offered the best combination of viewing angles and minimum drag. The Plexiglas hood was mounted in rubber in a metal frame for strength and rigidity required to avoid distortion and to prevent any binding of the canopy in the deep fuselage rails, one along each side of the cockpit and one on the centerline of the fuselage aft. The canopy was manually opened and closed by a hand crank.

    In order to accommodate the new hood, the rear fuselage had to be extensively cut down, but only minor re-stressing of the fuselage structure was necessary. The XP-51D made its first flight at Inglewood on Nov 17, 1943 with test pilot Bob Chilton at the controls. Having proven the concept, NAA diverted two P-51B-10-NAs [42-106539/106540] from the Inglewood production line and completed them as NA-106s with cut-down rear fuselage and bubble canopy, redesignated P-51D.

Another shortcoming of the P-51B was its limited firepower of only four wing guns. Also the guns were tilted and required a major kink in the ammo belt feeds, which caused frequent jams. NAA took advantage of their new Mustang to correct that problem with a redesigned installation of three .50 MG53-2 guns in each wing, all mounted upright and all fed by unkinked ammo belts. Inboard guns had 400 rpg and the rest had 270 rpg. Mustang users had the options of removing two of the guns and having just four with 400 rpg, and some pilots did actually select this option.

    A visible change introduced by the P-51D was in the increase of the wing-root chord. The landing gear was strengthened to accommodate the additional weight, but the wheels maintained the same diameter of 27". Wheel bays and doors were modified and the "kink" in the wing's leading edge made more pronounced than that of the P-51B. Four P-51D-1-NAs had been completed with the original B-type canopy before the first P-51D-5-NA model (NA-109) rolled off the production line. CBI aircraft usually had a direction-finding loop antenna ahead of the fin.

    Regarding the 85-gallon tank in the P-51B's rear fuselage and its adverse effects on the directional stability, things got worse for the P-51D. Cutting down the top line of the rear fuselage lost a lot of keel area, so to provide better directional stability a dorsal fin was added ahead of the rudder during production of the P-51D Block 10. Some earlier P-51Ds, and a few P-51Bs, were retrofitted with this fin, whose extra weight and drag was quite small.

    The P-51D/K introduced the K-14 gyro gunsight, based on a British Ferranti design and considered almost miraculous. A pilot needed only to dial in the wingspan of the enemy aircraft he was chasing and then feed in the target range by turning a handgrip on the throttle lever. Everthing was then done by an analog computer. All a pilot had to do was get the wingtips of his target lined up on the bright ring projected on the gunsight and press the trigger. This sight played a major role in the P-51D's impressive score of aerial victories. The K-14 was fitted almost from the start of P-51D production; the P-51K received the sight from mid-1944.

    Inglewood's 6,502 P-51Ds were the NA-109 (D-1 to D-10), NA-111 (D-15 and D-20), and NA-122 (D-25 and D-30). 1,600 P-51Ds were also constructed at the Dallas plant as NA-111 (D-5 to D-20) and NA-124 (D-25 and D-30). Most all Block-25 and subsequent Ds had underwing hardpoints for bombs, fuel tanks, and various types of rocket launchers, as well as "Bazooka" tubes in triple clusters. The Dallas plant also built 1,500 P-51Ks (also as NA-111), which had a slightly lesser performance than P-51D, and had an 11' Aeroproducts propeller in place of the 11'2" Hamilton Standard. Rocket stubs were introduced on the -10-NT and subsequent batches of the Dallas K production run.

    A total of 163 P-51Ks were completed as F-6K photo-recons. 126 Inglewood-built P-51Ds from blocks 20, 25, and 30 were converted after completion as F-6Ds and a few others were similarly converted near the end of the war. All of the photo Mustangs carried two cameras in the rear fuselage, usually a K17 and a K22, one looking out almost horizontally off to the left and the other one down below looking out at at an oblique angle. Most F-6Ds and Ks carried a direction-finding receiver and a rotating loop antenna.

P-51D [44-14017] was borrowed by the US Navy as [57987] to determine if it would be suitable for carrier-based operations, but was deemed unacceptable because of poor rudder control at low airspeeds at high angles of attack. Eleven Dallas P-51Ds were modified as TP-51D trainers with a second seat with full dual controls fitted behind the pilot [44-84610/84611, -84662, 45-11443/11450]. To fit the second seat, radio equipment had to be relocated. The bubble canopy was large enough to accommodate the extra seat. One of them was modified as a high-speed observation post for Genl Dwight Eisenhower to inspect Normandy beach-heads in June 1944.

    P-51Ds began to arrive in Europe in quantity in March 1944. The 55th Fighter Group (FG) was first to get them to replace its P-38s. The change from torqueless Lightning to the P-51 caused some initial problems, and directional instability took a lot of getting used to; however, once fully adjusted to their new mounts, pliots found that the P-51D possessed a marked edge in speed and maneuverability over all Luftwaffe piston-engine fighters above 20,000'. Luftwaffe pilots, conversely, considered the Mustang to be vulnerable to cannon fire, particularly the liquid-cooled Merlin engine which could be put out of action by even a lucky hit.

    The Mustang was the only Allied fighter with sufficient range to accompany bombers on their "shuttle" missions in which landings were made in Russia after deep-penetration targets had been attacked from English bases. The Mustangs also participated in low-altitude strikes on Luftwaffe airfields, a rather dangerous undertaking as these fields were very heavily defended by flak.

    In 1943 the Luftwaffe was planning to introducing jet-powered aircraft over Germany that posed a serious threat to Allied bombers and their escorting fighters. Mustangs first encountered Me.163 Komets on July 18, 1944 when a pair of them flew unscathed through a flight of P-51s. On Aug 5, 1944, Me.163s destroyed three bombers and shot down three escorting P-51s, but the next day a Mustang flown by LtCol John B Murphy of the 359th FG brought down a Me.163. Although the Komet gained much publicity and gave Allied high command much concern, it had extremely short endurance in the air and was dangerous to fly. It is doubtful that the Komets destroyed more than a dozen or so Allied aircraft during the entire course of the war.

    In Oct 1944 the Messerschmitt Me.262 began to appear in combat. The first jet kill by a Mustang was on Oct 7, 1944 when Lt Urban L Dreq of the 361st FiG shot down two Me.262s as they were taking off from their base. The Me.262 was nearly 100 mph faster than the P-51D, so to attack the jets in the air, a P-51 needed to dive in order to be able to close on the jets when they attacked the bombers. If attacked by an Me.262, the P-51 could easily turn and maneuver inside the enemy jet, placing itself in a position to meet the jet head-on or to get in a quick burst of gunfire if the enemy overshot. The Mustang was actually in a better position to defend itself in a dogfight with an Me.262 than it was able to fend off their attacks on bombers. It soon came clear that the best strategy was to jump them at their bases, since they were most vulnerable during take-off and landing. The favored tactic was for scores of Mustangs to circle high over known Me.262 bases, daring the jets to take off. If any rose to the challenge, diving Mustangs would be on them almost before their wheels were retracted. If they refused to take the bait, the bases would be strafed and the jets destroyed on the ground.

Several P-51 pilots became "ace-in-a-day" by scoring five or more victories during one mission—Lt William Beyer of the 361st FG, Capts William Whisner and Donald S Bryan of the 352nd, Lt Claude Crenshaw of the 359th, Capt L K Carson of the 357th, Lt J S Daniel of the 339th, Capt William Hovde of the 355th, and Capt "Chuck" Yeager of the 357th. Maj George Preddy of the 352th held the ETO record of six victories on one mission, Aug 6, 1944. Preddy was also the top AAF Mustang ace of the war, scoring 23.83 out of his 26.83 victories. Top-scoring Mustang-equipped fighter group was the 357th, with 609 air and 106 ground kills from Feb 11, 1944 to Apr 25, 1945.

    By the time Germany surrendered all Escort Groups of the 8th AF and some of the groups in the 9th AF had converted to P-51s. The RAF received 281 Ds and 594 Ks, designating them Mustang IV and Mustang IVA, but they did not enter service until Sep 1944, with Mustang IIIs still in active service until then.

    With the higher priority of the war in Europe, the P-51D did not arrive in the Pacific until late 1944. P-51Ds were initially based in the Philippines and on Iwo Jima, but by that stage of the war Japanese fighter opposition was rare, and Philippine-based Mustangs mostly performed ground-support duties. Still, while flying over enemy-occupied regions of Luzon on Jan 11, 1945, Capt William Shomo managed to down six Tonys and one bomber in one day while flying an F-6D photo-recon, for which he was awarded the Medal of Honor, becoming the second Mustang pilot of World War 2 to receive that award.

As resistance on Luzon ended, Philippine Mustangs bombed and strafed Japanese forces based on Formosa. Iwo Jima Mustangs flew the first escort missions with B-29s attacking Japan, and undertook the first land-based fighter strikes against Tokyo on Apr 7, 1945 when B-29s hit the Nakajima factories. Such missions involved flights of up to seven or eight hours at distances of over 1,500 miles. When Genl Curtis LeMay decided that most B-29 missions would take place at night from medium altitudes, the Iwo Jima Mustangs switched to ground-attack missions against Japanese airfields with generous use of the 5" rockets under each wing.

    The P-51D remained in service in considerable numbers with the USAF for many years after the war ended. On Sep 18, 1947 the re-formed USAF eliminated the P-for-pursuit category and replaced it with F-for-fighter—P-51 became F-51. Similarly F-for-photographic-reconnaissance was eliminated, and F-6D and F-6K aircraft became RF-51D and RF-51K; the two-seat F-6D became TRF-51D.

    In May 1946 the Air National Guard (ANG) was re-formed and fighter units received most of the F-51D/Ks from regular AAF service. It was agreed that the Mustang would go to ANG groups west of the Mississippi, with those east of the Mississippi getting the F-47 Thunderbolt. By Dec 1948 more than 700 P-51s and RF-51Ds were serving with 28 ANG squadrons.

    No fewer than 22 of the 27 ANG wings saw service in the Korean War that began on June 25, 1950. The Mustang was better suited to the small airstrips of Korea than were the F-80s and F-82s based in Japan. Japan-based F-51Ds were immediately sent to bases at Kimpo, Pusan, and Pohang in an attempt to halt the rapid North Korean advance. They were called on to carry the brunt of air support missions during difficult early days of the war, since jets of the time did not have the range to permit loiter time over a target.

To build up close-support forces, 145 F-51s were brought over on the carrier USS Boxer, then quickly assembled and flown to combat units. The 18th Fighter-Bomber Wing gave up its F-80s for F-51s, one of the few occasions in which a combat outfit traded its jets for piston-engined aircraft. The F-51s were instrumental in halting the North Korean advance, giving UN forces time to build sufficient strength to go on the offensive. Mustangs flew 62,607 tactical support combat missions while 351 were lost in action, most to anti-aircraft fire—it was not the best choice for low-level air-to-ground combat with a belly-mounted radiator and reliance on liquid coolant making it vulnerable to ground fire. Still, USAF Mustangs managed to down a few of the Yaks that made infrequent appearances. But when attacked by MiG-15 jets with superior performance, there was little the Mustangs could do but turn inside the MiGs, hit the deck, and run for home.

    The last active-duty Mustang was P-51D-30-NA [44-74936], finally withdrawn from service with West Virginia ANG in 1957. It is on display at the USAF Museum in Dayton, but painted instead as P-51D-15-NA [44-15174].

    POP: 800 P-51D-5-NA [44-13253/14052], 900 P-51D-15-NA [44-14853/15752], 1,000 P-51D-20-NA [44-63160/64159], 600 P-51D-20-NA [44-72027/72626], 1,600 P-51D-25-NA [44-72627/74226], 800 P-51D-30-NA [44-74227/75026]; (Dallas): 200 P-51D-5-NT [44-11153/11352], 400 P-51D-20-NT [44-12853/13252], 600 P-51D-25-NT [44-84390/84989], 200 P-51D-25-NT [45-11343/11542], 200 P-51D-30-NT [45-11543/11742]
    POP: 200 P-51K-1-NT [44-11353/11552], 400 P-51K-5-NT [44-11553/11952], 600 P-51K-10-NT [44-11953/12552], 300 P-51K-15-NT [44-12553/12852].

Engineering inspections were held in Feb 1944. The first XP-51F was flown by Bob Chilton on Feb 14, 1944. The second and third XP-51F flew on May 20 and 22 of that year. Equipped empty weight was about 2000# less than that of the P-51D, and combat weight was 1600# less. The engine was the 1695hp Packard Merlin V-1650-7, same as the P-51D. Considering that the equipped empty weight was about a ton less than that of the P-51D, the performance improvement was not as spectacular as might have been anticipated—max speed was 466 mph at 29,000'. Before construction began it was agreed that the last two NA-105 airframes would be fitted with Rolls-Royce Merlin 145M engines obtained from England under reverse Lend/Lease; those were designated XP-51G [43-43335/43336].

    Work on the conversion of the fourth and fifth NA-105 airframes as XP-51Gs began in Jan 1944 with the Merlin 145Ms. Five-blade propellers were fitted, but the ship was otherwise similar to the XP-51F. The date of the first flight of XP-51G is a matter of some dispute—most sources claim that first XP-51G was flown by Ed Virgin on Aug 10, 1944, but the manufacturer credits Bob Chilton on Aug 12, while still others say that Joe Barton may have taken the XP-51G up for the first time on Aug 9. The second plane followed on Nov 14 with the Merlin 145M driving a Rotol prop with five wooden blades (much like the Spitfire XIV). However, it flew only once with the Rotol in a 20-minute flight, and subsequent flying was with a more conventional Aeroproducts Unimatic A-542-B four-blade prop. It was readily apparent that it was the hottest Mustang yet—max speed of 472 mph at 20,750'.

    The third XP-51F was shipped to the United Kingdom on June 20, 1944 after preliminary flight checks. It was painted in RAF camouflage and was named Mustang V [FR409]. They found it to weigh only 7855# in interceptor trim and rated it very highly except for a severe lack of directional stability that required heavy application of rudder in certain flight conditions.

    The second XP-51G went to the RAF in Feb 1945, also named Mustang V [FR410]. It was reported to have achieved 495 mph in tests at Boscombe Down, although NAA claimed only 472 mph for the other G at the same altitude. However, by then RAF priorities had changed and no further flight testing took place, and its fate after the end of test flying is uncertain.

    Neither the XP-51F nor G ever went further than the prototype stage, but were players in the development of the P-51H. The Merlin 100-series of engines had not reached the stage where they were ready for production. Meanwhile Packard had set up its own development program and came up with the V-1650-9 version of its license-built Merlin, capable of delivering war-emergency power of 1900hp at 20,000' with water/alcohol injection. Packard said that they could deliver their new engine by late 1944, so it was chosen to power the next production Mustang, the P-51H.

The last prototype in the NA-105 series was the XP-51J, similar to F and G models except it re-introduced the Allison V-1710 to bring the Mustang full circle. The Allison V-1710-119 has a two-stage, gear-driven supercharger and was rated at 1500hp for takeoff and 1720hp with water injection at 20,700'. Unlike earlier Allisons, it had an updraft carburetor. Nose geometry was substantially modified, and all air inlets in the nose eliminated as carburetor air was taken in through a ram inlet at the front of the radiator duct and piped to the engine. And a dorsal fin was fitted.

    Two XP-51J prototypes were ordered [44-76027/76028]. The former made its first flight on Apr 23, 1945 piloted by Joe Barton. It weighed 6030# empty and 7550# normal loaded, and was anticipated that a max speed of 491 mph could be achieved at an altitude of 27,400', but this was never achieved during tests because the Allison had not been cleared for full-power operations. It was, in fact, loaned to Allison for use to iron out the bugs in their engine. The second prototype never flew, but was scavenged for spare parts to keep the first one flying. V-J Day brought all further work on the XP-51J to a halt.

    An odd fact is that no in-flight photos were ever taken of the XP-51F, G, or J—none has ever been found. It seems that pilots and other people were too busy with wartime testing to schedule photo sessions.

The first P-51H-1-NA was flown by Bob Chilton on Feb 3, 1945. 20 were built, all with the XP-51F tail. The distinctive taller tail was installed on the P-51H-5-NA and later production block aircraft and later retrofitted to earlier P-51H-1-NAs to eliminate the yaw instability that plagued earlier Merlin-powered Mustangs.

    Along with the P-47N, the P-51H was intended to be the leading AAF fighter used in a planned invasion of Japan. 2,000 P-51Hs were ordered—555 NA-126s and 1,445 NA-129s with minor differences—all to be built at Inglewood. 1,629 more examples were ordered from NAA's Dallas plant as NA-124, designated P-51M by the AAF. The P-51M differed primarily in having the V-1650-9A engine, which had a lower war-emergency rating by virtue of having the water injection deleted.

    P-51H [44-64420] went to the Navy in Aug 1945 for trials as a possible carrier fighter. Tests showed that the P-51H performed well, but since the war was over, and with jets on the horizon, a carrier-based P-51 was not further considered. A second P-51H [44-64192] also was used in 1948 for Navy tests of various aerofoil shapes at transonic speeds at Grumman Corp [09064]. After tests ended in 1952, the plane was transferred to an ANG unit. One P-51H went for RAF evaluation at Boscombe Down as [KN987].

    The last P-51H rolled off the production line in 1946. While too late to see action in Europe, by late summer of 1945 some had been issued to a few operational units, but those units were still in the process of working up to operational status when Japan surrendered. At V-J Day, 555 P-51Hs had rolled off the Inglewood production lines, but orders for 1,445 more were cancelled, along with the entire order for Dallas-built P-51Ms after only one example had been completed. Also cancelled was an order for 1,700 P-51L (NA-129), which was to be similar to the P-51H but with the 2270hp V-1650-11; none was built. Despite glowing pilot reports on its performance, because of its lighter structure it was not considered as being suitable for combat operations in Korea.

    POP: 20 P-51H-1-NA [44-64160/64179], 280 P-51H-5-NA [44-64180/64459], 255 P-51H-10-NA [44-64460/64714], 1 P-51M [45-11743].