The Bolkhovitinov S (Sparka – twin/joined (engine)) was a high speed bomber aircraft designed and built in the USSR from early in 1937.
To obtain high speed and good aerodynamics, it was powered by two M-103 inline engines in tandem, driving two contra-rotating, coaxial three-blade propellers.
The structure of the S was predominantly light alloy stressed skin, the wing having two spars with heavy flush-rivetted upper and lower skins, and the fuselage built up from pre-formed upper, lower, and side panels attached to four longerons to give a strong but relatively simple structure to build.
[1] Four 100 kilograms (220 lb) bombs were carried vertically in a bay between the pilot and the rear gunner-navigator, under a common canopy.
The plane had good handling, except for take-off and landing, which were difficult due to high wing loading.
[1] It was initially planned to also build a ground-attack version with two Ultra ShKAS machine guns in a rear part of fuselage, firing downwards and operated by the second crew member.
Also an interceptor aircraft was proposed, armed with 37 mm recoilless rifle, firing upwards at an angle, and again operated by the second crew member.