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Background

Red Air Forces considered the concept of a dedicated dive bomber since 1934, but only as a distant second alternative to tactical level bombers. In 1936 Nikolay Polikarpov initiated preliminary design for a three-seat SVB dive bomber powered with two M-100 engines. Prototype SVB drawings completed in February 1937 did not materialize, even in a prototype airframe; instead, Polikarpov reworked them into Polikarpov VIT-1 anti-tank ground-attack aircraft. Bombers were Tupolev's exclusive territory, and Polikarpov made a backup proposal to convert the VIT-1 airframe into a heavy fighter, code-named MPI (Russian: многопушечный истребитель, multi-cannon fighter). Air Forces rejected the VIT concept and advised Polikarpov to concentrate on the MPI heavy fighter. They made it clear that MPI was considered a technology demonstrator rather than a candidate for mass production.

VIT-1 to VIT-2

In July 1937 Polikarpov received a go-ahead to build two twin-engined fighters around M-100 engines. His factory in Khimki rolled out the first prototype of the original Polikarpov VIT-1 in heavy fighter configuration in October; flight test continued until February 1938 when VIT-1 was at last rejected, citing "failure to meet design targets".

On October 13, 1937 Polikarpov submitted a different proposal for a multi-role aircraft built around M-103 engines, Polikarpov VIT-2, that made possible three combat configurations: short-range, high-speed level bomber (BSB), anti-tank ground attack (VIT) and heavy fighter (MPI). The prototype, completed out on May 1938, was tested by Valery Chkalov and Boris Kudrin with M-103 engines, reaching 498 km/h level speed with 6166 kg takeoff mass (508 km/h with 5310 kg).

Polikarpov made a decision to replace M-103 with the new, more powerful M-105 that had not yet reached mass production - a decision that ultimately killed his twin-engined designs, including the SPB. Polikarpov's factory became a test and rework shop for the defective M-105s, but nevertheless VIT-2 took off in August 1938, showing only a marginal increase in maximum speed (to 513 km/h). Independent tests in Zhukovsky revealed that even this number was inflated by 15 km/h. A redesigned VIT-2, delivered in February 1939, barely made 500 km/h - the minimum design target set in 1937 yet far ahead of any competing designs and mass-production models.

Naming ambiguity

The Commissariat for Aircraft Industry suppressed Polikarpov's project, to the point where it banned presentation of VIT-2 to Air Forces generals. M. Nersisyan, engineer in charge of VIT-2, managed to get through to Kliment Voroshilov to get a permission for a show flight. Air Forces chief Yakov Smushkevich, fascinated by the show, took over the sole prototype for a thorough examination by Air Forces test pilots. Two runs of test flights, in September-October 1938 and in February 1939, persuaded Smushkevich in the feasibility of a fast dive bomber, thus VIT-2 program was renamed SPB (Russian: skorostnoy pikiruyushchy bombardirovshchik, high-speed dive bomber). Polikarpov disagreed and proposed a different acronym, SBP (Russian: skorostnoy bombardirovshchik, pikiruyushchy, high-speed bomber with dive capability). His design was optimized for speed; conversion to a true dive bomber required structural reinforcement that, Polikarpov feared, will lead to unacceptable loss of speed. For a brief period in the first half of 1939 the three names - SBP, SPB and VIT-2, were used concurrently; by the end of 1939 SPB prevailed.

Producing the prototype

March 29, 1939 Vyacheslav Molotov authorized small-scale production run of SBP (sic) at the Plant No. 124. Polikarpov objected: his prototype needed substantial changes, and the chosen plant was not capable of building advanced aircraft. Production was moved to Plant No. 22, then building Tupolev SB; Polikarpov was ordered to deliver SPB (sic) based on revised VIT-2 on Janyary 1, 1940 and a dedicated PB-1 dive-bomber, with different engines, on July 1, 1940. What looked like a successful contract became an end of Polikarpov's firm. In November 1939, while Polikaprov toured German aircraft facilities, Commissar for Aircraft Industry Mikhail Kaganovich created a new design firm, headed by Artyom Mikoyan (later Mikoyan-Gurevich OKB, today's Mikyan) and transferred 80 of Polikarpov's staff to Mikoyan. Polikarpov continued working without a proper production workshop and without his key personnel, managing to roll out the first SPB prototype (numbered SPB 1/0, zero standing for "prototype") on the last day of 1939. On December 14, 1939 he submitted another proposal based on the same airframe - Polikarpov TIS, a heavy escort fighter (on the same day Mikoyan proposed a competing Mikoyan-Gurevich DIS).

Polikarpov's other competitors produced their own prototypes for fast bombers: Kocheryegin OPB, Petlyakov 100, Tupolev 103, Yakovlev BB-22, Arkhangelsky B-2. A state commission held on November 18, 1939 reviewed these prototypes and recommended Petlyakov (first choice, although expensive) and Yakovlev for mass production, yet somehow failed to notice already existing SPB.

Crashes and inquiries

The second prototype SPB (SPB 2/0) was completed on January 10, 1940. SPB 1/0, piloted by Boris Kudrin, flew on January 18; SPB 2/0 on March 26, the first serial production SPB 1/1 on April 17 and SPB 2/1 on April 26.

On April 27, 1941 serial SPB 2/1, piloted by Pavel Golovin, crashed in an unrecoverable spin after a thirty-minute level flight around the Khodynka Field. All SPBs flown to this date suffered from troubles with M-105 engines, usually ending in bursts of motor oil. On the previous day Golovin, who flew the same aircraft from the factory field to Khodynka, reported increased (but not critical) temperature of the right engine. Investigators did not even consider engine failure; their inconclusive ruling blamed the spin either on insufficient longitudinal stability of the aircraft on crossing the cloud front, or the pilot error. A contemporary review of the crash by Perov and Vladimirov named engine failure the most likely cause of the crash. The commission allowed further tests of SPB 2/0, 1/1 and 3/1 and grounded SPB 1/0l Polikarpov was ordered to run a full set of wind tunnel tests at TsAGI and obtain a TsAGI review of the aircraft stability.

Soon after the crash Smushkevich reported to Stalin that the choice of production aircraft, either SPB or 100, will be made "in the nearest days" of May 1940. The opinions of Air Forces generals were changing rapidly from full support to SPB on May 15 to rejecting it on May 25. By the end of May the decision was made against Polikarpov, although the tests continued. On June 2, 1940 test pilot Mikhail Lipkin experienced another in-flight engine failure; landing with his right engine out, he clipped a parked Tupolev SB and crashed but survived. Test continued; in the same month Polikarpov obtained an order for fifteen more serial aircraft although his OKB kept on losing engineers and workers to other firms.

Another crash occured on June 30, 1940, killing Lipkin and his flight engineer. Lipkin was instructed to test the airframe for wing flutter at 600 km/h. Withesses on the ground saw the aircraft disintegrating in midair, in level flight at an apparently high speed; the investigators blamed the crash on wing flutter and personally on Polikarpov's deputy Zhemchuzhin who did not adjust, as instructed, trim tabs on the ailerons. TsAGI commissioners believed that SPB was prone to flutter upon reaching 500 km/h. Yet twenty days later TsAGI issued a report stating that SPB could not succumb to flutter until reaching 800 km/h. Engine and airscrew failures, again, were not considered at all.

On July 29, 1940 the new chief of aircraft industry Alexey Shakhurin grounded all test flights of SPBs. Polikarpov was cut off the information loop, denied access to investigation reports. By the end of August Shakhurin has written off the project completely; the sole surviving SPB was donated to Moscow Aviation Institute, the factory tooling reused for Pe-2 production.

Sources

• Vladimir Perov, Nikolay Vasilyev (in Russian). Zagadka SPB ili pochemu pikiruyushchi bombardirovshchik ne poshol v seriyu («Загадка СПБ, или почему пикирующий бомбардировщик не пошел в серию»). Krylya Rodiny magazine, June 1997.