The SPB first flew in July 1937, with TB-3 piloted by Stefanovskiy, and I-16s piloted by Nikolayev and Taborovskiy. Following the successful test program in 1938, Zveno-SPB was accepted into service. By 1 February 1940, Soviet Air Force was supposed to receive 20 TB-3s and 40 I-16s, with the same number going to the Soviet Navy. Vakhmistrov was also asked to investigate the possibility of using Tupolev TB-7, Tupolev MTB-2, and GST (PBY Catalina) as the motherships, as well as arming I-16s with 500 kg (1,100 lb) bombs. By 1939, the government support for the project had waned, the Navy canceled all of its orders, and the Air Force reduced the number of fighters from 40 to 12. However, Soviet military observers noted the success of the Luftwaffe Junkers Ju 87 dive bombers in the opening stages of World War II. As the Soviet Union had no dive bombers, it was decided to resume low-scale work on the Zveno-SPB. Testing of the first production Zveno began in June 1940. It differed from the prototype in using the much more powerful I-16 Type 24 fighters. A total of six mothership-fighter combinations (six TB-3s and twelve modified I-16 Type 24s) were completed. All were attached to the 2nd Special Squadron of the 32nd IAP (Fighter Regiment) of the 62nd Aviation Brigade of the Black Sea Fleet Air Force stationed in Yevpatoria(Crimea). Mirroring the nickname of the Zveno experiments, the squadron was dubbed "Shubikov's Circus" (Цирк Шубикова) after its commander Arseniy Shubikov.