Under that is what used to be Plant 2, birthplace of the 314 Clippers, the XB-15, thousands of B-17's, the B-47, B-52, and the first three 737's, and now being demolished

. Everything was well hidden in plain sight, even the runways and ramps by clever application of paint that made the field look like a neighborhood development with 'streets' and 'parks' until some Army bean massager decided the field needed barrage balloons, they were put out and the field then stuck out like a flashlight in a dark basement.
Salvagers of wooden beams are paying huge money for the old growth beams in the building, some four feet thick X 2 feet wide.
The lighter colored sawtooth buildings in WALLYB's link were Jorgenson Steel and out the top of the picture (South) was Kenworths original plant (the very dark structures built out of dark, dark red bricks), the MoF is a bit outside the top of that photo, and the 1st. South bridge in the photos is being removed and replaced. In spite of what the text says, no B-29's were built @ BFI, it was the delivery point and production flight test center for the 29 since Renton is just over the hill and pretty small as far as ramp space.