This is my first design to have custom retractible landing gear and doors. I was unable to synchronize the extension process, but the retraction is satisfactory.

AG-1 - Nose .30 cal ShKAS machine gun
AG-2 - Dorsal .30 cal ShKAS machine gun
AG-3 - Ventral .30 cal ShKAS machine gun

History:
Based on the unbuilt Tupolev ANT-36 (DB-1) single-engined bomber, the ANT-37 was a twin-engined monoplane of stressed skin, dural construction, fitted with a high-aspect-ratio wing and tailwheel landing gear, the main units retracting into the engine nacelles. Powered by 800 hp (597 kW) Tumansky M-85 radial engines, the prototype first flew on 16 June 1935, but crashed on 20 July 1935 when the tail broke off resulting in one of the crew being killed. A second prototype-with a reinforced fuselage-was then built, but it flunked state trials due to severe flutter, with the program then being given up, with plans for production cancelled. However, a third machine was completed as the "ANT-37bis" for setting records -- being stripped of armament, given with additional fuel tanks, and fitted with Tumansky M-86 radials providing 860 hp (640kW) driving three-bladed variable-pitch props. It was given the name “Rodina” (Motherland), and used to set a women's distance record. It remained in service into 1943.

DB-2

Cockpit of DB-2

Three-view drawing of DB-2

Inboard profile of DB-2

Size comparison of DB-2 (ANT-37) to SB-2 (ANT-40) and T-1 (ANT-41)