A number of parasite fighter experiments were carried out at NII VVS (Scientific-research Institute of Red Army air force) between 1931-39. Vladimir Sergeevich Vakhmistrov was a designer and project engineer who suggested the use of air-launched fighters in order to improve their range and payload. Number of fighters (both biplanes and monoplanes) were either attached to a bomber on the ground or engaged and disengaged in-flight. In cases where fighters were mounted on the ground, take off was made fighter engines set at full power.

Zveno-1 December 3, 1931
Involved Tupolev twin-engined TB-1 aviamatka (mother-ship) and two specially modified Tupolev I-4 biplane fighters mounted above the wing of the bomber. The test took place over Monino with Zelevsky piloting TB-1, Chkalov and Anisimov at the controls of I-4s and Sharapov and Vashmistrov as observers. Both fighters were released simultaneously at altitude of 3,000 meters. Despite of the number of the mishaps the idea was found feasible and experiments were continued.

Zveno-1A September 1933
Consisted of Tupolev TB-1 mother-ship flown by Stefanovskii and two Polikarpov I-5 biplane fighters mounted on above the bomber's wing. I-5s were piloted by Grozd and Kokkinaki.

Zveno-2 (or 2A) August 1934
Consisted of Tupolev TB-3 four-engined bomber piloted by Zalevskii and three I-5 fighters flown by Suzi, Suprun and Alt'nov. Two I-5s were attached on the top of the bomber's wings and third mounted above the fuselage. All three "took off" from bomber successfully.

Using ramps I-5 fighters were rolled onto the TB-3 wings and fuselage on the ground. Of course there were no means to retrive fighters after launch in mid-air.

Zveno-3
Consisted of a Tupolev TB-3 bomber and two Grigorovich I-Z fighters suspended by a special attachment under the wings of the TB-3. Grozd and Korotkov piloted the I-Z fighters. Test flight ended tragically - Korotkov was killed in a collision with the wing of TB-3.

Zveno-4 Unknown

Zveno-5 April 1935
Single I-Z fighter was suspended under TB-3. The large steel-tube trapeze mounted under the fuselage of the bomber and hook-equipped fighter was able to engage and disengage in flight. Test pilot Stepanchenok was able to perform the maneuver. The rendevue took place at 1,000 m above Monino airdrome. I-Z disengaged after few moments - TB-3 was not capable of landing with fighter underneath. TB-3 was flown by Stephanovskii and I-Z by Stepanchenok. Final hook-on was completed successfully on 23 March 1935 - first underfuselage hook-up in the world.

Mid-air rendezvous of I-Z and TB-3.

Zveno-6 August 1935
Involved a TB-3/AM-34 with two Polikarpov I-16 suspended between two large V-strut links under the wings of the bomber. Bomber was flown by Stefanovskii and fighters by Budakov and Nikishin.

Zveno-? November 1935
Culmination of the parasite fighter experiments was this Zveno consisted of two I-5 fighters fixed above and two Polikarpov I-16 mounted beneath the wings of TB-3/AM-34. The bomber was modified with under fuselage trapeze so that I-Z canon fighter piloted by Stepanchenok was hooked in flight. This combination consisted of six (!) aircraft of four various types. All fighters were released simultaneously. I-5s were flown by Nikashin and Suprin.

Zveno-SPB July 1937
Two Polikarpov I-16 tip 5 dive-bombers were suspended under the wings of the TB-3/AM-34RN with Stefanovskii at the controls. Each I-16 flown by Nikolayev and Taborovskii carried two FAB-250 250 kg bombs and were not able to take off under its own power with both full fuel load and 500 kg payload. The takeoff weight of this Zveno was 22,000 kg and top speed reaching 268 km/h. Fighters were using the fuel of the mother-ship up to the moment of disengagement. Upon release I-16 within 50 km of target fighters were supposed to return to the closest base. This zveno was adopted by Morskaya aviatsiaya of the VMF for anti-shipping operations. The combat unit was based at Yevpatoria equipped with six modified TB-3/AM-34RN and 12 SPB.
SPB was used operationally in WWII from Crimean bases against ships, bridges and other targets till 1942. Most famous combat mission took place on August 25, 1941 against Dunabe bridge at Chyernovod, Romania (railroad bridge to Constanta). The mission was successfully completed and bridge was destroyed.

Zveno-7 November 1939
Same as previous entry but three I-16 tip 5 fighter-bombers were used instead. Flown by Stefanovskii, Suprun and Nyukhtikov.

Soviet Aircraft and Aviation 1917-1941 by Lennart Andersson, 1994.
The Encyclopedia of Russian Aircraft 1875-1995 by Bill Gunston, 1995.
Encyclopedia Aviatsia. Edited by Svischev, 1994.
Strange Planes Series: Parasites. Discovery channel video.