Staff Writer (Updated: 5/11/2016):
The XP-58 was designed as a two-man twin-boom design with powerful armament and even more powerful engines. The system would have the pilot at front, operating the aircraft and firing an impressive array of 4 x 37mm cannons which were originally just a twin set of .50 caliber machine guns. Later designs actually took into account the mounting of a massive 75mm cannon it place of the quad cannon mounting. At rear, the gunner would sit and operate two rear-facing .50 caliber heavy machine guns installed in the booms to ward off any rear-approaching enemy fighters. Engine power would be supplied from untested powerplants as developed by the Continental company. After much going back an forth on specifications and capabilities between the Army and Continental, the powerplant development was already in jeopardy. Even when a Pratt & Whitney powerplant was selected to replace the Continental design, Pratt & Whitney resources were allocated to pressing radial designs elsewhere leaving Lockheed to fit the Wright R-2160 Tornado radials into their airframe.
Though a much powerful engine, the Tornado forces Lockheed engineers to rework virtually every internal system of the XP-58, delaying the project even further but alas the US Army was happy with the Wright engine capabilities.
With the added power, the Army and Lockheed now both began salivating at the added armament capabilities afforded to the ever-increasing power of the XP-58. As such, the original twin .50 calibers mounted in the nose gave way to the aforementioned 4 x 37mm cannon array. Further twin .50 caliber machine gun mounts could be added in a top and bottom turret assembly increasing the forward firing damage ten-fold. The system was becoming quite capable in downing a bomber or fighter in a single shot!
As with any high-altitude aircraft, the issue of cabin pressurization was addressed. Couple the weight of such a system with the weight of the added cannon armament and new engines and the XP-58 weight began to soar to new heights (no pun intended).
When it appeared that the hordes of enemy bomber formations infiltrating the American skies would never materialize, the US Army came back to Lockheed and ordered a reclassification of the XP-58 Chain Lightning as a close-support strike aircraft. The 37mm cannons would neatly fit this bill except that the aircraft's size had ballooned considerably, making it an enticing low-flying target. The complex substructures were also noted as being quite fragile, meaning the slightest damage from small arms fire or flak could easily spell doom for the craft and its pilot.
As such, the XP-58 was once again redesignated as a bomber-killer. Further developmental and production delays doomed the Tornado powerplants and Allison V-3420 (twin V-1710s joined) liquid-cooled radials were fitted instead. Flying on D-Day itself, the XP-58 Chain Lightning made its maiden voyage over California. By all reports, the system proved quite stable in handling. Nonetheless, the complicated internal workings, delayed production and development and the propensity of the turbosuperchargers to catch fire did the massive program in. After barely 20 such flights, the XP-58 was dead.