From an article in the Huffington Post by Doug Bradley:
Donut Dollies were single, female college graduates who were used primarily as morale boosters for U.S. combat troops in Vietnam. Many of these young women were as motivated by JFK's call to duty and service as we guys were. But their training manuals instructed them to present themselves as reminders of girlfriends, wives and sisters waiting back home. As one SRAO member told my friend Heather Stur, author of Beyond Combat: Women and Gender in the Vietnam War Era, the donut dollies were to be "nonsexual symbols of purity and goodness." Good luck with that in a combat zone!