Tensions escalated in the region and fighting broke out again in the Khalkin Gol, eastern Mongolia, close to the border with China, in May 1939. At first there were two SBAP (Russian acronym for 'Fast Bomber Regiment') units present; 150 and 38 SBAP.
In this battle, which lasted until 16th September and caused Soviet forces heavy losses, the SBs were not involved at first. However, due to the increasing Soviet casualties, two regiments of SB bombers were deployed in June, flying their first mission of the 26th. The SBs were employed thoroughly when the Japanese launched an attack in early July.
Both 150 and 38 SBAPs were composed of a mixture of old and new models of the SB, so the differing speeds between bombers proved to be a serious problem when maintaining formation, specially when the Japanese Nakajima Ki-27 fighter attacked and the poor defensive arrangement of the SB could do little to defend, as the radio operator had to man both dorsal and ventral turrets. In order to minimise casualties against Japanese fighters, Soviets changed their tactics, so the SBs had to fly in altitudes over 6.100 m (20.000 ft), where it was difficult for the Japanese to intercept. In August a new regiment, 49 SBAP arrived in the battlefield and the SBs were employed in aerial raids against Japanese forces until Soviet and Mongolian forces lead by Georgy Zhukov carried out an offensive which ended with the cease-fire in 16th September 1939.
The SB was also employed by the VVS in the Soviet Invasion of Poland. At 6:00 am, on 17th September 1939 approximately 310.000 Soviet troops crossed the Polish border from the east and the SB was the backbone of the VVS during this campaign. Both Belarussian and Ukrainian fronts had a total of 637 SB 2M-100As available when the Polish-Soviet War broke out. They were assigned to 10th, 16th, 18th and 70th SBAPs.
During the first days of the invasion, most of their missions were against strategical railroad stations and communication hubs, including the Stanislawow and Buczacz marshalling yards, as well as Polish airfields of Kowel, Dubno and Luzk, where they encountered resistance and Polish P-11 fighters shot down two SBs that day.
On 19th September, some of the remaining P-11s tried to attack a formation of SBs without any success at all.
The SBs also raided one of the last Polish strongholds near Kamien Koszyrski on 22nd September. By that date the Polish Air Force had already collapsed and the VVS only flew some sporadic sorties over Poland. On 29th September a formation of various SB 2M-100As bombed the town of Nujno.
VVS's bombing operations over Poland were limited until 6th October 1939, when the Polish campaign ended. This was because of the lack of suitable targets and only a handful of SBs were lost in the 20 days this campaign lasted.
The Tupolev PS-40 was a civilian transport variant, which we will cover in a future post.
The Tupolev MMN was an version which featured several aerodynamical improvements. It was made on the basis of the SB 2M-103 and was powered by two 1.100 hp Klimov M-105 engines. Each engine, housed in an aerodynamically modified cowling was equipped with a centrifugal two-speed twin-stage supercharger driving a three-bladed propeller.
The wingspan was reduced and the tail was redesigned and reduced in size. Nose section was also modified in order to have clearer portions. Armament consisted on two 7,62 mm ShKAS machine guns placed in a dorsal turret and a single ShKAS machine gun in the nose section. This reduced the defensive armament of the regular SB as they normally had two ShKAS in the nose.
The MMN also featured additional wing fuel tanks and a second landing light. It was also lighter than the SB 2M-013.
It was accepted for state trials in September 1939 and achieved a top speed of 458 km/h (285 mph) flying at 4.200 m (13.780 ft). This was only 8 km/h (5 mph) faster than the standard SB 2M-103. As it had smaller wings, it also had smaller wing area, resulting in a faster landing speed of 160 km/h (99 mph), making it more dangerous. The MMN did not pass state trials because of its nimble defensive armament, its marginal faster speed and its high landing speed over the SB 2M-103. The only prototype was assigned to Aeroflot where it received the nickname of Shchuka (Pike).
Sources:
1st https://en.wikipedia.org/wiki/Tupolev_SB
2nd https://en.wikipedia.org/wiki/Battle_of_Lake_Khasan
3rd https://en.wikipedia.org/wiki/Battles_of_Khalkhin_Gol
4th https://j-aircraft.com/research/George_Mellinger/sovietorderofbattle.htm
5th Signal Squadron - Aircraft In action 194 - Tupolev SB in Action
6th https://en.wikipedia.org/wiki/Soviet_invasion_of_Poland
7th http://www.airvectors.net/avtu2.html
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