Here is one source: http://www.essortment.com/all/slipperyelm_rdbk.htm

Here is the quote from that page:

“Two hundred years ago, George Washington and his army faced defeat due to the need of food for the survival of his troops. Slippery Elm fed the starving troops. They stayed alive by making and eating a Slippery Elm Gruel. Gruel is kind of porridge made from pounding the inner bark of the
tree into a powder then mixing it with water.

General Washington needed medicine for the soldiers. He mixed the powder with a little water to make poultices and salves to heal the soldiers’ gunshot wounds.”