I found the following paragraph in a National Materials Advisory Board report:
To minimize structural weight and thus maximize payload capability, the Air Force elected to use 7178-T6 aluminum in the lower wing skins as well as in other locations in the aircraft along with 7075-T6 aluminum. The commercial 707 used 2024-T3 aluminum in the lower wing skins at about two-thirds the stress level.
https://www.nap.edu/read/5917/chapter/6#87