In late 1942, a deal was worked out between Britain and the USA in which Spitfire VBs would be transferred to the 8th Air Force in England, mainly for use as fighter-trainers. This cleared the way for Lend-Lease supplies to continue of the new Mustang model to the RAF.
The RAF equivalent to the USAAF P-51B/C was known as the Mustang III. The RAF ultimately received 274 P-51Bs and 626 P-51Cs. RAF serials were FB100/FB124, FB135/FB399, FR411, FX848/FX999, FZ100/FZ197, HB821/HB962, HK944/HK947, HK955, HK956, KH421/KH640, SR406/SR438, and SR440. Serial numbers FX848, 849, 907, 909, 910, 911, 913, 914, 915, 916, 918, 927, 928, 932, 948 were handed back to the USAAF upon arrival in Britain. HK944/947, 955, 956 were ex-Twelfth USAAF aircraft. KH490 crashed in the USA before delivery. Serial numbers SR406/438 and SR440 were a mixed bag of P-51Bs and Cs delivered to the RAF from the USAAF--US serial numbers were respectively 43-12162, 43-12407, 43-12412, 43-12473, 43-12484, 43-12427, 43-70114(?), 43-12189, 43-12177, 43-7039, 43-6831, 43-12155, 43-12188, 43-12456, 43-12480, 43-12399, 42-10663(?), 42-106683, 42-106630, 42-106687, 43-7071, 43-7144, 43-5595, 43-7171, 43-6829, 43-12420, 43-7152, 43-7135, 42-103209, 42-106478, 42-106431, 43-7007, 43-12420, 43-7159. (Question marks denote serial numbers which are probably erroneous). The first RAF squadron to receive the Mustang III was No. 65 Squadron based at Gravesend, which received its planes in December 1943.
A total of 59 Mustang IIIs were diverted to the Royal Australian Air Force and to other Allied air arms.
After these Mustang III aircraft had been delivered to England, the RAF decided that the hinged cockpit canopy offered too poor a view for European operations. A fairly major modification was made in which the original framed hinged hood was replaced by a bulged Perspex frameless canopy that slid to the rear on rails. This canopy gave the pilot much more room and the huge goldfish bowl afforded a good view almost straight down or directly to the rear. This hood was manufactured and fitted by the British corporation R. Malcolm & Co., and came to be known as the "Malcolm Hood". This hood was fitted to most RAF Mustang IIIs, and many USAAF Eighth and Ninth Air Force P-51B/C fighters received this modification as well.
Many pilots regarded the Malcolm-hooded P-51B/C as the best Mustang of the entire series. It was lighter, faster, and had crisper handling than the later bubble-hooded P-51D and actually had a better all-round view. Its primary weakness, however, was in its armament--only four rather than six guns, which often proved prone to jamming. Some of the modifications applied to the P-51D to improve the ammunition feed were later retrofitted into P-51B/Cs, which made them less prone to jamming. With modified guns and a Malcolm hood, the P-51B/C was arguably a better fighter than the P-51D, with better visibility, lower weight, and without the structural problems which afflicted the D. Its departure characteristics were also more benign.
The first RAF base to receive Mustang IIIs was at Gravesend in Kent. The Mustang III initially equipped No. 65 Squadron in late December of 1943, followed by No. 19 Squadron in March of 1944. Later the Mk. III also equipped Nos 64, 65, 66, 93, 94, 112, 118, 122, 126, 129, 165, 234, 237, 241 249, 250, 260, 268, 306, 309, 315, 316, 345, 430, 441, 442, and 516 Squadrons and No. 541 Squadron of RAF Coastal Command. These units included four Polish squadrons (306, 309, 315, 316), three RCAF, and one Free French.
The new RAF Mustang IIIs began operations late in February 1944, escorting US heavy bombers as well as both US and RAF medium bombers.
Numerous RAF Mustang IIIs were diverted to the interception of V-1
"buzz-bombs". Some of them were "souped up" by using a special
high-octane fuel and internal engine adjustments in order to increase
the intake manifold pressure and made it possible to achieve a speed
of 420 mph at 2000 feet. Since the typical V-1 flew at 370 mph, this
made the "souped-up" Mustang very useful against these weapons.