Complex sentence: Based on the unbuilt Tupolev ANT-26 (DB-1) single-engined bomber, the ANT-37 was a twin-engined monoplane, fitted with a tailwheel landing gear, the main units retracting into the engine nacelles.
Split into simpler sentences: Based on the unbuilt Tupolev ANT-26 (DB-1) single-engined bomber the ANT-37 was a twin-engined monoplane. Fitted with a tailwheel landing gear, the main units retracting into the engine nacelles.