Bones, the factory worker autographed B 25 because it was the 1,000th and last B-25 made at the Inglewood, California, plant.

B-25 Mitchell assigned to the 12th BG (M) was the 1,000th and last B-25H (S/N 43-5104) manufactured at the North American Aviation (NAA) Inglewood, California, plant. As this B-25H was special, USAAF commanding general "Hap" Arnold permitted those who worked on the airplane to write their names on it.

Sometime during this autographing, it was nicknamed Bones. Later, while at Columbia Army Air Base, South Carolina, the B-25 was assigned its first crew, and it departed from Morrison Field at West Palm Beach, Florida, on November 11, 1944. On December 1, Bones arrived in Feni, India, where it was assigned to the 12th BG's 82nd Bomb Squadron. Through the month of December, Bones was involved in several successful missions against enemy targets.

Toward the end of WW II, Bones had a low-altitude midair with a large vulture that went through the right windshield and just missed the copilot. The crew was forced to return to base. The War ended a few days later, and it was never flown in combat again. Soon after V-J Day, a salvage crew was assigned to prepare Bones for the long flight home. They balked at the fetid smell inside it; apparently, no one had cleaned up the vulture.

 the 1,000th and last B-25H (S/N 43-5104) manufactured at the North American Aviation (NAA) Inglewood, California, plant.
https://www.flickr.com/photos/dougsheley/3294474530/