The lasting legacy of the Northrop XP-56 "Black Bullet" isn't its configuration but rather how the development of the aircraft changed the way metal alloys were used in aircraft and that in turn, changed the way metal alloys were handled in other industrial applications outside of aircraft manufacturing. At the time Northrop got the contract to develop the XP-56, aluminum alloy production was insufficient to meet forecast demands for just the aircraft industry alone in the 1939-1940 timeframe. Many manufacturers turned to alternative materials such as Duramold, wood, and even stainless steel.
Jack Northrop, however, always had an interest in advanced materials applications besides unconventional aircraft configurations. While working for Douglas in the 1930s, he got interested in the use of magnesium sheet but because it was a relative unknown in the industry of the day, he got little support from Donald Douglas. When he left Douglas to start his own company in 1939, use of magnesium sheet at the time was considered unworkable as it couldn't be riveted with aluminum alloy rivets due to galvanic corrosion and welding was difficult at best.
But magnesium was abundant in land deposits and seawater and its 2/3 lighter weight than comparable thickness aluminum alloy sheet allowed thicker skins and parts for an equivalent weight. With welding seen as the only viable way of working with magnesium sheet in an aircraft, Northrop hired an associate from Douglas, Vladimir Pavlecka, to develop a way of efficiently and cleanly welding magnesium. The end result of Pavlecka's work resulted in Heliarc welding and the patenting of the Heliarc welding torch which quickly found industrial applications outside of aircraft manufacturing. Northrop and Pavlecka's Heliarc welding eliminated the use of flux or a flux-based electrode as in conventional welding- use of flux on magnesium caused a weak and porous weld seam. Using a tungsten electrode, a stream of helium was used to eliminate atmospheric oxygen from the local environment at the tip of the torch, resulting in a high quality magnesium weld.
On the XP-56, the assembly jigs were used to hold magnesium sheet in place and then the interior was welded to the sheet's inside. So quite literally, the aircraft was built from the outside to the inside! With no rivets, the skin of the XP-56 was very smooth and so stiff that the aircraft didn't even need main wing spars in its design.
In someways developing Heliarc welding was the easiest problem to solve- Northrop's engineers devoted considerable effort into getting good quality magnesium sheet that was flat and the stout assembly jigs and welding fixtures were a first in any industry.
Source: American Secret Pusher Fighters of World War II: XP-54, XP-55, XP-56 by Gerald Balzer. Specialty Press, 2008, p111-126.