After Japan's attack on Pearl Harbor on December 7, 1941, the United States entered World War II. Due to America's entry into the war, a new need for human power and the production of all manner of materials cropped up. Because many able-bodied white males were being called off to fight (African Americans still experienced significant discrimination in the armed forces at this time), this created employment opportunites for both women and African Americans.
Women began working in factories that made ammunition and other war materials. Thus, the image of "Rosie the Riveter" was born. In 1941, President Roosevelt ended discrimination in hiring practices in the defense industry by establishing the Fair Employment Practices Committee, which paved the way for African Americans to gain employment in places like factories, shipyards, and steel mills.
Because segregation and discrimination were still present in the armed forces at this time, African American men and women had a more difficult time than their white counterparts. Pressure, though, from civil rights groups eventually led to African American nurses being allowed to serve in England by 1944. Also by 1944, African American fighter pilots, who had to train separately from white pilots, started flying missions.
Thus, World War II helped open the way for both women and minority groups to prove that they could serve American in a variety of jobs that had never been opened to them previously.