It was an extremely advanced design for the period, with a single-unit steel "bathtub" running from just behind the propeller to the rear crew position acting not only as armour, but also as both the main fuselage structure and engine mounting in one unit. Engine access was provided by armored steel panels, one on either side of the nose. The armour was 5 millimetres (0.20 in) thick and weighed 470 kilograms (1,040 lb). It protected the crew, the engine, the fuel tanks, and radio equipment, when fitted. The flight control surfaces were connected to the aircraft's controls by push-rods and bellcranks – not with the usual steel cable control connections of the era as push-rods were less likely to be severed by ground fire.
The aircraft had two fuel tanks with a total capacity of around 32 US gal. The main tank (divided into two parts for redundancy) was supplemented by a smaller, 7.9 US gal "gravity tank". This was intended to supply fuel to the engine by gravity feed in the event of an engine fuel pump failure; it contained enough fuel for thirty minutes on full power. There was a manual fuel pump for use when the gravity tank became exhausted.
The aircraft could be fitted with two downward-firing machine guns for ground attack, but they were found to be of limited use because of the difficulty of aiming them. The J-Is were mainly used for army co-operation and low-level reconnaissance. They were also used for dropping ammunition and rations on isolated or cut-off outposts that could not be easily supplied by other means.