Critically-acclaimed director and writer Tyler Perry has created the film for Netflix, bringing the women's achievements to a global audience.
"They were tasked with delivering 17 million pieces of mail that had been just thrown into hangers, that had been mouldy and a lot of the names on the letters were illegible," he said.
Viewers are left in no doubt about the challenge the women faced and the fact that others had failed the same mission.
Major Adams declares in the film: "They have only given us a six-month ticking clock and I thought that was a long time until I realised the magnitude of this situation.
"When there is no mail there is low morale."
They were the first and only Women's Army Corps unit of colour to be stationed in Europe during World War Two.
The battalion was mostly black women but there were also women of Caribbean and Mexican descent.
"There's that great line that Tyler has written for Charity Adams in the film where she says, 'we are fighting a war on every front,'" Washington said.
"Because it just felt like, yes they were dealing with World War Two but they were also dealing with racism and misogyny.
"And even when they came back home they were dealing with people being prejudiced against them because they went into the army and people couldn't believe that they had accomplished this incredible goal, so there were rumours that they had only been sent there to be concubines for the black male soldiers.
"So that's why a lot of the women never talked about their legacy, didn't share this heroic moment in history with anybody."
The women cleared the backlog in three months - half the time they'd been given.
Charity Adams was the first black woman to command an overseas battalion in the US Army.
Washington said she was full of admiration for the character she plays.
"People talk about loving her, respecting her because even though she was strict and stern and tough they knew that she wanted the best for them and that she loved them," she said.
"And I think that's such a unique leadership quality, the kind of leader who brings out the best in you because they believe in the best in you."
The 6888 film is released on Netflix on Friday, December 20.