Could it be that the paint was to reflect the thermal radiation from a nuclear detonation?

The undersides of the fuselage and wings of some B52s were painted in gloss white anti-flash paint to prevent damage caused by the detonation of its own bomb. Probably, not much need for that in most active conflicts since the late 60s.

That makes sense, theannoyingnoob and perseus.

The reason they stopped painting many of the planes during WW2 was that it could cut several hundred of pounds of the weight of the plane and being shiny and lighter they could get another 5 mph out of the plane. It may not sound much but it could be the difference of living or dieing. Also with the bombers they only managed a few missions before they were lost so why waste time painting them, Also the contra trails given off by B17s and the others were like giant fingers pointing out where the planes were for the Ack Ack gunners, you could not miss them.

Oh yeah. It was simply lack of paint, not shiny paint! Somehow I imagined unpainted planes to look rusty, like unpainted ship-hulls