SPB, V.S.Vakhmistrov/ N.N.Polikarpov/ A.N.Tupolev

20k b/w "History of aircraft construction in the USSR", Vol.1 p.506

Experiments with parasite fighters (see Vakhmistrov's Zveno article) continued with considerable success during 10 years. Finally was found that the 'lower' configuration is the most practical. In this case (Zveno-6) two I-16 were attached under the wing of carrier bomber, outboard from the outer engines.

Unusual practical application of Zveno-6 was the Composite Dive Bomber, Sostavnoj Pikirujuschij Bombardirovschik. It was a TB-3 heavy bomber carrying two fighter under the wings. Fighters were attached while on the ground, than their landing gear was retracted. Procedure required minimal jacking. Each I-16 carried in turn up to 1000kg bombs. I-16 were released near the target, performed high precision dive bombing and return to the base on their own. This allowed to combine long range of heavy bomber with high-speed strike by small and agile fighters.

First flight was performed in July 1937 by pilots Stefanovsky (carrier), Nikolaev and Taborovskij (I-16s).

At least six SPB were built, and used (mostly) early in the Second World War on Black Sea Theater. More than thirty missions were performed during 1941. Last (known to me - A.S.) sortie was flown (according to Shavrov) on May 25, 1943 against Dunabe bridge at Chyernovod. This bridge had very strong anti-aircraft defense system, and few attacks by Il-4 failed. Then single SPB was sent. After approaching undetected from the sea, TB-3 released I-16s. Those approached bridge at high speed and altitude... and destroyed the target fifth their four FAB-250 bombs. All three aircraft returned safely.