The Model 15 (PW-9) was the first successful Boeing-designed fighter and established the company as a major American builder of military aircraft. Its internal bracing was arc-welded tubing rather than the spruce and wire used in older biplanes. However, it still had wooden spars and ribs. Boeing also introduced the first oleo shock absorbers for aircraft. It was designated PW-9 (for "pursuit water-cooled design 9) by the Army and designated FB-1 by the Navy. Between 1923 and 1928, Boeing built 157 PW-9/FBs in different versions, as well as 77 derivatives as NBs (Navy training planes). The NBs were 4 feet longer than the PW-9/FBs and powered by either 180- or 200-horsepower engines. Boeing sold five NBs to Peru.