Douglas B-18 Bolo, part one

 

The Douglas B-18 Bolo was an American medium bomber which served with the United States Army Air Corps (USAAC) during the late 1930s and early 1940s. 

In 1934 the USAAC requested a bomber with double payload and range than the Martin B-10 which, was just entering service as USAAC's standard bomber. The next year an evaluation was carried out at Wright Field, Ohio, where Douglas showed their DB-1 design and competed with the Boeing Model 299 (which would eventually be improved into the Boeing B-17 Flying Fortress) and the Martin 146. 

Boeing's candidate was clearly superior, however, as pilots forgot to unlock controls when taking off causing the prototype to crash, it was removed from consideration and its high price per unit ($58.500 vs $99.620 of the Model 299 - $1.105.142 and $1.881.953 adjusted for inflation in the past year) also played a big factor in making the DB-1 as the chosen bomber. Therefore the DB-1 was ordered into mass production in January 1936 as the B-18.

The DB-1 design was basically the same as the DC-2 with many modifications. The wingspan was 4.5ft - 1.4m larger, the fuselage was deeper to better fit bombs and the six-men crew, the wings were fixed in the middle of the cross section instead of the bottom because of the deeper fuselage. It was equipped with three turrets, placed in the nose, ventral and dorsal positions, with this last one being retractable. Each turret was equipped with one Browning 0.3in machine gun. It could also carry a normal payload of 2.000 lb (910 kg) of bombs, however, it could be overloaded up to 4.400 lb (2.000 kg) at maximum. 

Sources:

1. https://en.wikipedia.org/wiki/Douglas_B-18_Bolo

2. https://www.valka.cz/Douglas-B-18-t72780

3. http://www.aviation-history.com/douglas/b18.html