The San Diego Air and Space Museum has posted a great many images to Flickr, including a fair amount of concept art. One piece (you can see the best-rez version HERE) depicts a Ryan Aeronautical concept for an air boat equipped with a Flex-Wing. Date would probably have been around 1962, plus or minus a few years. The exact role here is a bit obscure… why would the Army have boats like this? Perhaps a troops transport of some type, though a helicopter would probably be a better choice.