The P-51D had several design improvements over previous versions of the Mustang, and nearly 8,000 were built, the most of any type of P-51. It featured a "bubble-top" canopy that gave the pilot much improved visibility, and new K-14 gunsight, six .50-cal machine guns (improved from the prior 4 machine guns), and a simplified ammunition feed system that greatly helped to reduce gun jams.