Boeing’s first all-metal monoplane military aircraft was called the B-9.
The Boeing B-9 was a twin-engine bomber developed for the United States Army Air Corps in the early 1930s. It used an all-metal structure and a low-wing monoplane layout, features that represented a major shift away from the biplanes and mixed construction common in earlier military aircraft.
The B-9 was fast for its era because of its streamlined design and retractable landing gear. Boeing produced several related versions, but the type did not receive a large production order. Its development nevertheless helped Boeing gain experience with modern metal monoplane construction.
The B-10 followed as a more capable and successful Boeing bomber. The P-26 was a fighter rather than a bomber, while the B-17 came later and belonged to a much heavier generation of aircraft.