Proving the Flying Fortress As a result of the General Staff's decision, only 13 YB-17s and one YB-17A were ordered by mid-1937 as compared to 133 B-18s and 177 B-18As. The Air Corps was determined to prove the value of the four-engine B-17 with this handful of service test planes. It flew its YB-17s at every opportunity over the U.S. and even scheduled good will flights to South America to demonstrate the long-range capabilities of the plane. It eventually proved its point, and by the end of 1938, contracts for 39 additional B-17Bs were awarded. In addition, a new four-engine bomber, the Consolidated XB-24, was ordered on March 30, 1939. At the time World War II began in Europe in September 1939, the first B-17Bs were beginning to come off the production line. Click here to return to the Four-Engine Bomber Overview.