TK-1, TK-4 by V.F.Bolkhovitinov

In the 1930-1940 several design teams came up with idea of a new class of aircraft - "Tjazholij Krejser" (Heavy Cruiser). This had to be a modification of production bomber with enormous defensive firepower. It supposed to fly in formation with (similar in design) bombers and repel attacking fighters.

V.F.Bolkhovitinov developed two modifications of the DB-2A to comply with this idea, but none was built.

And for good - later in WWII some USAF B-17's were tried same way - with increased firepower and ammo. Experience was negative, because Luftwaffe pilots easily 'calculated' such a "Heavy Cruisers" and brought them down first, leaving the rest of bombers (with armament decreased in favor of bomb load) unprotected. Only conventional fast long range escort fighters could provide bombers with reasonable protection.

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References:

"History of aircraft construction in the USSR", Vol.2 p.27