The P-51D was considered by many as the definitive Mustang version. One of the problems with the Mustang was poor pilot visibility to the rear. The British had field-modified some Mustangs with fishbowl-shaped canopies called Malcolm Hoods, which somewhat corrected the problem. All British P-51s were equipped with Malcolm Hoods, and some American ones were also equipped with them. However, North American decided to do better than that, and equipped the new P-51D Mustang with teardrop-shaped bubble canopies derived from the British Hawker Typhoon fighter. The fuselage behind the pilot had to be cut down to accommodate the new canopy, but the new design offered the pilots unrestricted vision in all directions with virtually no distortion. A common misbelief is that cutting down the rear fuselage to mount the bubble canopy reduced stability, requiring a dorsal fin to be added to the forward base of the vertical tail. Actually, both the B/C and later D/K models had a low speed handling problem that could result in an involuntary "snap-roll" under certain conditions of air speed, angle of attack, gross weight, and center of gravity. Several crash reports tell of P-51Bs and Cs crashing because the horizontal stabilizer was torn off during maneuvering. The report says: "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" Photographs of P-51B/C models fitted with a similar dorsal addition, which has to be made to fit the different contours of the B/C rear fuselage, prove that the problem requiring it wasn't unique to the D/K versions. While some existing aircraft do not have the dorsal extension fitted, many were equipped at some point in their service or refurbishment with a taller model tail, which provides a similar increase in yaw stability. Also, civilian owned examples often have newer, lighter radios, an absence of external munitions and drop tanks, removed guns and armor plate, and an empty or removed fuselage tank - reducing the need for the dorsal fin. Among other modifications, armament was increased with the addition of another two M2 machine-guns, bringing the total to six. The inner four machine guns had a supply of 400 rounds and the outer two 270. In previous P-51s the M2s were mounted at angles that led to frequent complaints of jamming during combat maneuvers. The new arrangement allowed the M2s to be mounted in a more standard manner that fixed a lot of the jamming complaints. The .50 caliber machine guns, though lacking penetrative power, proved adequate against the FW-190 and Bf-109 fighters that were the main opponent of the USAAF by this time. Later models had under-wing rocket pylons added to carry up to 10 rockets per plane. The P-51K differed from the P-51D only by its hollow-bladed Aeroproducts propeller. This propeller turned out to be badly manufactured and created dangerous vibrations during times of high throttle, and was eventually replaced. The P-51D/K started arriving in Europe in the middle of 1944 and quickly became the prime USAAF fighter in Europe. It was produced in larger numbers than any other Mustang variant. However, by the end of the war, roughly half of all operational Mustangs were still B or C models. P-51Ds were also built under licence in Australia by the Commonwealth Aircraft Corporation

(Information extracted from Wikipedia)