How did American aviation solve the shortage of escort fighters over Germany?

A single-engine, high-speed fighter with the range of a bomber was considered an engineering impossibility. Bomber defense was initially organized in layers: the shorter-ranged P-38s and P-47s escorted the formations during the early stages of the raid and handed off to the P-51s when they were forced to turn back.

Ronald Harker, a Rolls-Royce test pilot, suggested fitting a Merlin 61. That engine carried a two-stage supercharger with an intercooler. The results were so positive that North American began converting several aircraft, work that led to the P-51B/C model (Mustang Mk III), the first long-range fighter able to match the Luftwaffe's fighters.

With external fuel tanks, the Mustang could escort bombers from England to Germany and back. In August a P-51B was tested with an additional 184 US-gallon (700 L) internal tank, but longitudinal stability problems appeared. When the Pointblank offensive resumed in early 1944, the situation had changed.