The C-97 was the cargo version of the World War II B-29 bomber, distinguished by an expanded fuselage. Its first flight came in 1944, and it became especially valuable to the U.S. Air Force after the tanker version was introduced in 1950, using flying boom technology for faster fuel offloads. Of the 890 aircraft built, 816 were delivered as KC-97 tankers. The final model, the KC-97L, added two jet engines to increase speed so it could refuel newer jet bombers and fighters. Most KC-97G and KC-97L aircraft served almost exclusively in the tanker role, with rear clamshell doors replaced by the boom operator's position and observation windows. The last KC-97s were retired from the National Guard in 1977. One KC-97 acquired from Beale AFB in California was extensively restored for display.