Please enjoy ANT-21 and ANT-21bis pictures.
Model picture shows twin vertical tail stabilizer and closed cockpit.http://aviadejavu.ru/Site/Crafts/Craft30920.htmhttp://www.tupolev.ru/ant-21-mi3
"The Tupolev ANT-21 was a Soviet twin-engined four-seat heavy fighter, which also had the designation MI-3 (Mnogomestnyi Istrebitel – Multi-seat fighter). It was not accepted for production, only two prototypes being built."General characteristics
Crew: 3, Length: 11.57 m (38 ft 0 in), Wingspan: 20.76 m (68 ft 1 in), Wing area: 59.2 m2 (637 sq ft), Empty weight: 4,058 kg (8,946 lb), Gross weight: 5,463 kg (12,044 lb), Powerplant: 2 × M-34N liquid-cooled V12 engine, 610 kW (820 hp) eachPerformance
Maximum speed: 350 km/h (217 mph; 189 kn) at 5,000 m (16,400 ft), Service ceiling: 8,300 m (27,231 ft)Armament
Guns: 1 × 20 mm cannon in flexible mounting in nose, 2 × fixed forward-firing 7.62×54mmR PV-1 machine gun and 2 × Degtyaryov machine guns in dorsal position
"The new aircraft, the ANT-21bis or MI-3D (Doubler – understudy) was completed in April 1934. Despite the new tail, it suffered tail vibration at certain power settings which resulted in the addition of tail bracing struts.
" (As you can see.)