The Internet provides several sources. I've never been sure. I lean toward the Seabee origin.
The Seabees of World War II tells how the Bobcats, the first unit sent overseas, struggled against unimaginable odds to build a fueling station at faraway Borabora...and takes the reader to the other side of the globe where Seabees floated two artificial harbors across the English Channel for the invasion of Normandy. One of their proudest mottoes was:
"The difficult we do immediately. The impossible takes a little longer."
The Difficult is that which can be done immediately;
the Impossible [is] that which takes a little longer.
The difficult we do immediately. The impossible takes a little longer.
Art Berg says his mother said it, as in:
Mr. Berg tells the story of how his life was changed by the words of his mother soon after he learned he would be dependent upon others for the rest of his life. Her advice and encouragement for him was that the difficult takes time and the impossible takes a little longer.
Art Berg broke his neck in an automobile accident in 1983 and has been inspiring people ever since as a world class wheelchair athlete, young entrepreneur, motivational speaker and author. He has trademarked the phrase The Impossible Takes a Little Longer. http://www.artberg.com/speakingtopics.html
"The difficult we do immediately,
The impossible takes a little longer."
~Samuel C.Florman, "The Existential Pleasure of Engineering"