Why did the DC-3 have wing sweep? When looking at the silhouette of a DC-3, one can clearly see that the wings are swept. This design is from 1935, before the high-speed research work on wing sweep was ever implemented, and the DC-3 was not very fast anyhow.

Leading edge sweep was 14 deg and trailing edge 0 deg, so the quarter chord wing sweep was 10.5 deg, not inconsiderable, and causing some of the ugly stall behaviour associated with wing sweep. A 1938 NACA video of a DC-3 stalling here, a bit lengthy but it illustrates the tip stalling first, then the aileron.

None of the benefits and some of the drawbacks - why did the DC-3 have wing sweep?

Image Source
The wing was swept to move the center of lift back, closer to the center of gravity. When the design of the DC-1 was advanced to a point where the center of gravity became clear, a redesign of the wing-fuselage intersection would had been more effort, so the outer wing panels were swept back a little.

From this website by Jeff Lucker:


  the unique and characteristic swept wing of the DC-3 was an attempt to
  keep the wing in its original location as the center of gravity moved
  toward the rear.


When Douglas produced new variants (DC-2 and DC-2 sleeper, a.k.a. DC-3), they kept the proven layout.