Why are the B-47’s inboard and outboard engines mounted so differently? The B-47 has six underwing GE TG-190 engines, two on each inboard pylon and one on each outboard pylon.

The inboard engines are mounted on a long pylon that extends well below and forward of the wing, such that the inboard-engine tailpipes are (longitudinally) just about even with the wing’s leading edge.

The outboard engines, in contrast, are mounted on much shorter pylons and fit snugly against the underside of the wing, with their tailpipes extending aft of the wing’s trailing edge; they’re placed so far back that their pylons even protrude beyond the trailing edge.



(Image originally by the United States Department of Defense, via Balcer at Wikimedia Commons; modified by me.)

Why the long pylons and forward mounting for the inboard engines, versus the short pylons and rearward mounting for the outboards?
The forward pylon mounting of most jets with wing mounted engines is to use the engines' mass to move the overall center of mass of the wing forward, closer to or ahead of the torsional axis of the wing box.  Boeing discovered this helps with aerolasticity related issues with long, flexible, swept wings.  A bit like mass balancing a control surface, but on the root structure.  

The outermost engines, being so close to the tip where the chord is short, have to be farther aft or they would move the wing's center of mass too far forward in that area.