This article from the Wichita Eagle covers the release of the 10,000 Boeing/Stearman Kaydet training airplane and the B-29 "X" airplane. Both airplanes had their production numbers painted on their fuselage to represent their respective milestones in aircraft production. The "X" on the B-29 denoted the fact that the official production numbers for the B-29 were classified during World War II.
Kansas Memory
Kansas Historical Society
10,000th Kaydet is delivered Friday to armed forces