Beginning in the 1930s the basic pillbox design was employed by milliners, or hatmakers, who created a new style of head covering: the pillbox hat, a smallish, brimless round hat that featured straight sides and a level top. Pillbox hats were popular because of their simplicity and elegance. They most often came in solid colors and were usually unadorned with accessories except for a colored net veil, or a single pin or jewel. They were, however, made of an array of materials, some of which were elaborately designed. These included green wool with ornate gold cording; black velvet, smothered in black beads; and white organdy, a transparent fabric, with attached overlapping organdy petals and silk rose bouquets. Pillbox hats might also be made out of the furs of mink, lynx, fox, or leopard skin. 

Musician Bob Dylan (1941–) incorporated the image of the latter into a song about a jilted lover, "Leopard-Skin Pillbox Hat" (1966)

— Encyclopedia of Fashion

The popularity of the pillbox hat increased during the post–World War II (1939–45) era and reached its peak at the 1961 inauguration of President John F. Kennedy (1917–1963), when his wife, Jacqueline Kennedy (1929–1994), wore a simple, unadorned bone wool pillbox hat designed by Halston (1932–1990). Previously, Mrs. Kennedy did not favor hats of any kind, but she was so taken by Halston's design that the pillbox hat became her trademark. She even was wearing a pink one on November 22, 1963, as she cradled her husband in her arms moments after he was shot while riding in a Dallas, Texas, motorcade. The cheerful femininity of Jackie Kennedy's pink suit and pillbox hat are ironic reminders of that tragic day.