Boeing gained valuable early experience in metal structures in the 1920s by rebuilding surplus de Havilland DH-4s, replacing their wire-braced wooden fuselages with welded steel tubing. From 1923 to 1925, 186 airplanes were rebuilt using the Boeing-developed arc welding process for steel-tube fuselages. These aircraft were designated DH-4M (for modernized). The arc welding process later was crucial development of the first Boeing-designed fighters.