10/31/2017. Remarks by Johan Visschedijk : "Thirteen improved B-299s (serials 36-149 to 36-161 c/n 1973 to 1985) were ordered on January 17, 1936, to service test the new bomber design and one additional airframe was ordered for static test purposes. The military designation of YB-17 had been assigned when the contract was drawn, but this was changed to Y1B-17, indicating procurement from F-1 funds, on November 20, 1936. This designation change so soon before the first flight of the aeroplane has produced an erroneous impression that there were two separate series of service test B-17s, especially since many official photographs and documents used the original YB-17 designation long after the change was made. All of the thirteen 299Bs were officially Y1B-17s before the first was flown on December 2, 1936. The designation was changed to plain B-17 at the completion of the service test period.
Outwardly, the Y1B-17 was identical to the B-299. The major change was from the single-row Pratt & Whitney S1E-G Hornet (R-1690) engine to the very similar Wright SGR-1820-39 Cyclone and the crew was decreased to six. Minor changes were made in armament details and in the undercarriage. All Y1B-17s were delivered between March 1 and August 5, 1937."