Just to clarify, the copy was not of the B-52 Stratofortress, but of the B-29 Superfortress. It was indeed reverse-engineered from three airframes:

• B-29-5-BW s/n 42-6256 Ramp Tramp of the 771st BS
• B-29-15-BW s/n 42-6365 Gen. H.H. Arnold Special from the 794th BS
• B-29-15-BW s/n 42-6358 Ding Hao from the 794th BS

Andrey N. Tupolev's OKB-156 was tasked with designing a manufacturable copy of the B-29, initially designated B-4. It was, to quote Yefim Gordon, 'a monstrous task.' Stalin promised to provide Tupolev with any resources he might need and set the deadline in two years' time, i.e. summer 1947.

The new bomber bas was to be a carbon copy of the Superfortress. As per Stalin's orders, no deviations were to be tolerated, and this would indeed explain the isue of the rogue rivet (which is almost certainly a myth or something that originated as a joke). To produce the copy, some companies were assigned to this task and this task only, still others were simply created from the ground up just to work on a certain part of the aircraft, such as the radio equipment and so on. One of the most difficult tasks was copying the alloy of the airframe itself. Metallurgists had to calculate the exact method of making it by infering all the details from parameters such as tensile strength and hardness of actual B-29 parts.

The difference between the imperial and metric system were problematic. And it has to be said that the Soviets did a phenomenal job overcoming this obstacle. The fuselage skin was made of D16AT duralumin. The empty weight of the finished aircraft was 35,270 kg (the B-29 weighed 34,930 kg, the difference was below 1%!). You might find different figures though. Graham Simons states that 'the Tu-4 weighed about 3,100 lb [1,400 kg] more than the B-29,' which is still a more than decent result.

Manufacturing of proper quality rubber and plastic proved a challenge to the Soviet industry as well, and Tupolev wouldn't budge when he was asked, numerous times, to agree on the use of somewhat lower quality substitutes.

However, not everything was copied. The engines are the one notable example of giving up on ccopying altogether. Instead of copying American engines existing Soviet ones were adapted for this purpose, the 18-cylinder air-cooled radial Shvetsov ASh-73TK, a derivative of the Shvetsov M-25 used in the old I-16 fighter.

Also, the IFF system was not copied (which is somewhat obvious) and a more up-to-date short-wave command radio was copied and fitted. Moreover, the defensive armament was substituted with Soviet-made 20 mm and later 23 mm cannons. The fire control system, however, was a copy.

There are also some humorous obstacles (not so funny for the engineers, though). To quote Gordon again:

One item in the American manuals puzzled the Soviet engineers mightily. It was a phrase reading 'Start the putt-putt'. None of the engineers could recall coming across this term before; the team researched much aviation literature but to no avail. The mystery was solved by pure chance when someone started the emergency generator/auxiliary power unit which was driven by a two-stroke engine and this started emitting an unmistakable putt-putt-putt-putt-putt...

Eventually, the Soviets achieved a level of mastery with the design. They developed quite a number of unique variants of the Tu-4, comparable with the number of variants of the B-29 itself. There was even ana an airliner, the Tu-70, though only a prototype was built.

All in all, the copy was very precise, but not really a carbon copy. It had some alterations where necessary and the engineers didn't copy the bomber blindly, even though that qas was Stalin's order, esentially. It was a blatant copy, true, but there was no stupidity in its design.

Sources:

• Yefim Gordon, Vladimir Rigmant, Tupolev Tu-4: Soviet Superfortress. Midland Publishing 2002.
• Graham M. Simons, B-29 Superfortress: Giant Bomber of World War 2 and Korea. Pen & Sword Aviation, 2012.