Starting in 1942, the British had a promising new American-built fighter: the North American P-51 Mustang I. This aircraft was developed specifically for the British, and they were first to use it in combat.
The first two RAF Mustang I squadrons, 26 and 239, formed in early 1942. These Mustangs had square-tipped wings, boxy cockpits and Allison engines rated at 1,200 hp. They lacked the powerful 1,500 hp Merlin engines that started with the faster, higher-flying, longer-range P-51 Mustang B and C and the signature bubble glass added starting with the P-51 Mustang D. Regardless, the new Mustang I fighters were good, and their pilots were optimistic.
Notes Tom Booth, "Prior to the introduction of the "teardrop" canopy on the P-51D variant, a number of P-51B's & C's were fitted with the [Spitfire's] bulged "Malcomb" hood. This still proved inadequate and the pilots wanted greater visibilty like that which existed on the Hawker Typhoon's "blown" canopy. The teardrop canopy was introduced on the P-51D-NA model in July 1944.
To fit the teardrop canopy, a section of the aft fuselage behind the cockpit had to be cut down.The area lost was compensated for by adding the "dorsal fin" build up in front of the tail section."