What are the unusually-enlarged wing sections on this P-38 Lightning?
It was a test bed that was looking into laminar flow on the wings. 


  was converted in 1942 as a two-seater, with an elongated central
  nacelle extending aft of the wing trailing edge, intended as a
  research vehicle to find ways of reducing drag, and was the only P-38
  to have have a full dual set of flight controls. Later it was modified
  with enlarged laminar-flow wing sections outboard of the engine booms,
  complete with slots and boundary layer control by means of exhaust
  bleed air.


There is some discussion on it here as well.