Staff Writer (Updated: 2/2/2015):
While World War 2 had officially begun in September of 1939, the United States did not formally enter the war until late-1941 following the Japanese attack on Pearl Harbor. The nation mobilized the following year and its large war industry ramped up to meet demand. The initial primary focus was on the European Theater leaving forces in the Pacific to made due for the interim - the survival of Britain and the Soviet Union were key factors in the demise of the Axis powers in Europe. Once the situation across the Atlantic had stabilized, American attention then turned to the West and the Empire of Japan where territory now spanned across the Pacific Ocean. Each holding was essentially helped by long distances of open water which would force invaders to commit considerable naval assets and manpower in forcing Japanese defenders from their entrenched positions. Such offensives - through "Island Hopping" - would eventually claim the lives of millions of participants.
One of the unique challenges facing American warplanners was the range required to support ground, naval and aerial actions. While the Boeing B-29 was slated to bomb far-off targets (including Japan proper), it required capable fighter escorts with similar long-range qualities. Long-range qualities in aircraft required sufficient fuel stores and multiple crewmembers to share the workload. In 1943, North American Aviation started development of a possible contender for the long range escort role, taking its excellent P-51 Mustang fighter as a starting point. It was deemed that the principle qualities of the base P-51 could be largely retained though expanded upon by way of simply joining two P-51s as one. Modifications would seem relatively minor but proved rather deep to the point that the resulting product was considered an all-new aircraft. This involved the joining of the inboard wing surfaces (the main span and the tail), concentration of armament in a center section, new landing gear arrangement and altered cockpits with redundant flight controls and systems. The primary pilot would be seated in the left fuselage cockpit with the navigator/co-pilot in the right fuselage cockpit - the workload being theoretically shared between the two men. The use of two powerplants ensured a failsafe should one engine fail over the unforgiving Pacific waters. From this thinking was born the North American NA-120 proposal which was approved by the United States Army Air Force (USAAF) on January 7th, 1944, giving rise to two prototypes (s/n 44-83886 and MX-485) intended to test the viability of the project. North American workers christened their unique concept as the "Twin".
The prototypes were collectively designated as "XP-82" and the powerplant of choice became the Packard series inline piston engine, license-produced versions of the excellent British Rolls-Royce Merlin Vs. The engine proved a heady performer and, with two units coupled, offered double the power output. As each unit drove their own propeller assembly, "torque" (the natural occurring pull of the airframe in one direction caused by the spinning propeller blades) was neutralized by assigning the other propeller to spin in the opposite direction. Each engine was to drive a four-bladed propeller.