The first 747-100s had six windows on the upper deck. Later, airlines began to use the upper deck to hold more people instead of using it as a lounge. Boeing then made an upper deck with ten windows. Some −100s had the new ten-window upper deck installed after the plane was made (retrofitted). The −100 had Pratt & Whitney JT9D-3A engines. Boeing did not make a freighter version of the 747–100, but some airlines have turned some 747-100s into freighters. 167 747-100s were made.