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?

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    $\begingroup$ Notice how the trailing edge is at a right angle to the fuselage, which simplifies the aileron design. My first instinct says the design was settled on a tapered wing and they had to choose which wing edge would be at a right angle to the fuselage, and went with the trailing one. Overall that amount of sweep is neglegible. $\endgroup$ – AEhere Aug 25 '17 at 6:35
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    $\begingroup$ Many other aircraft had tapered wings at the time. The DC-3 had 14 deg leading edge, 0 deg trailing, resulting in 10.5 deg at quarter chord line. The Fokker 100 has 15 deg at quarter chord, flies at Mach 0.77. $\endgroup$ – Koyovis Aug 25 '17 at 6:45

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.

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    $\begingroup$ Why did they do that, instead of putting a plug in front of the wing, I wonder. $\endgroup$ – Koyovis Aug 25 '17 at 12:11
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    $\begingroup$ @Koyovis: At those times the fuselage was aerodynamically shaped and not meant to be extended. Every bulkhead was different (a tradition which became a liability for the Constellation later). Only with pressurization and the rationalization in production during the war did the preference shift to cylindrical fuselages. $\endgroup$ – Peter Kämpf Aug 25 '17 at 17:57

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