Armament consisted of 2 x 0.30 Browning M1919 medium machine guns in fixed, forward-firing mountings along the upper frontal portion of the fuselage. This could be replaced by a combination arrangement of 1 x 0.30 caliber machine gun with 1 x 0.50 caliber Browning heavy machine gun. Variants arranged to carry bombs did so through an external system and this could vary based on model number and customer requirement.
The Boeing aircraft was produced under a variety of known variant designations. XP-12 became the USAAC evaluation version of the F4B-1 and fitted the R-1340-7 engine of 450 horsepower - nine of which were produced. The P-12B was a single example with a special engine cowl and R-1340-9 engine of 525 horsepower. 96 examples of the P-12C emerged and utilized a ring cowl design with modified "spread-bar" undercarriage. 35 examples of the P-12D followed with R-1340-17 series engines of 525 horsepower. The P-12E used a semi-monocoque fuselage structure with all-new vertical tail surface and 110 were produced (some with tail wheels replacing original skids). The P-12F was produced in 25 examples with the R-1340-19 engine of 600 horsepower. P-12J was a converted one-off from the P-12E line and fitted the R-1340-23 engine of 575 horsepower.
Concerning the F4B marks, 27 F4B-1 models were produced for the US Navy and featured a bomb rack under the fuselage.F4B-2 utilized the spread-bar landing gear arrangement with tail wheel and 46 of the type were completed. F4B-3 was the F4B-2 though with a semi-monocoque structure and 21 were produced. The F4B-4 was the F4B-3 with new tail fin design, the R-1340-16 engine of 550 horsepower and provision for 2 x 116lb drop bombs. Some of these also featured stowage for an onboard life raft.
Brazil (Model 256/267) became an export customer of the F4B fighter as did China (Model 218), the Philippines, Spain and Thailand (Model 100E). Over two dozen were produced for export. American aviation pioneer Howard Hughes was proud owner of a special two-seat commercial variant (Model 100A) of this Boeing design though this airframe was later converted back to its original single-seat form.
The F4B/P-12 retained a relatively long service life considering the aviation advancements made throughout the 1930s and 1940s. It was a primary pursuit mount for the US until the mid-1930s when replaced by the newly-arriving P-26 "Peashooter" but still served in training roles into 1941. The final F4B was retired by the Brazilian Air Force in 1949 while more than a handful have existed as museum showpieces.
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