The first Y1B 17 (36-149) flew for the first time on December 2nd 1936. She were different from the prototype by her Wright-GR 1820-39-Cyclone radial engines with 694 kW (930 hp), places for a nine-headed crew and some insignificant detail changes full set up. Between January and August 1937 12 bombers were submit to the 2. bomb group of the US Army Air Corps out on Langley Field in Virginia.
The 13th production model convicted for the further prooftesting to Wright Field. After one of the Y1B-17 had come through the turbulenzen of a hurricane, decided however, also to complete the aircraft as a use bomber provided for statics tests.
This aircraft (37-369), equipped with GR 1820-51 per 746 kW (1015 hp) and Moss/general Electric waste gas turbocharger, received the name Y1B-17 ares of (model 299 F) and started to the maiden flight on April 29th, 1939. The following prooftesting convinced the US Army air corps of the superiority of the loader engine in comparison with the piston jet engine ventilated normally. Therefore was such explains way of stand to engines to this for all future versions of the Flying Fortress.