Tupolev's bureau had been sketching pressurised, long-range bombers throughout the war, and a home-grown design would eventually have flown. But an indigenous aircraft meant years of prototyping, testing and teething troubles at a moment when the United States already had an atomic bomb and the means to deliver it. Copying the B-29 collapsed that timeline. It also solved a subtler problem: it forced Soviet industry to master a complete, coherent, state-of-the-art aircraft rather than a patchwork of promising but immature national technologies.

The cost was pride and, arguably, innovation - Tupolev is said to have resented spending years reproducing another nation's work. But the strategic calculus was sound. Within a few years the USSR had a fleet of modern bombers and, just as importantly, factories that knew how to build them.

The single most-quoted detail of the Tu-4 story is the aluminium. American sheet came in imperial thicknesses; the nearest Soviet metric gauges were slightly thinner or thicker. Simply substituting them would have thrown off the aircraft's weight and structural margins, so engineers had to recompute the affected structure. Similar puzzles arose across the airframe, from fasteners to wiring to instruments calibrated in different units.

This is why the popular image of blind, slavish copying is misleading. Reproducing the B-29 in the Soviet Union was not photocopying - it was a vast exercise in translation, in which tens of thousands of small engineering decisions were made to reach the same result with different materials and standards.

It is worth pausing on the sheer coordination involved. Engine plants, instrument makers, radio and radar workshops, metallurgical institutes and the airframe bureau all had to converge on a single coherent design, to a common set of tolerances, at the same time. Programmes of that complexity routinely fail. That the Soviet Union pulled this one off in only a couple of years, starting from three foreign bombers, remains one of the great feats of twentieth-century industrial organisation - regardless of what one thinks of copying a rival's aircraft.

You will often read that Soviet engineers copied a repair patch, a bullet hole, or an antenna scar from one of the interned bombers onto every production Tu-4. It is a marvellous illustration of over-literal copying - and it is almost certainly folklore. No reliable source documents it, and it sits uneasily with the genuine, well-attested differences between the two aircraft.

Those real differences matter more than the myth: Shvetsov ASh-73TK engines rather than Wright R-3350s; ten 23 mm NS-23 cannon rather than Browning .50-calibre machine guns; Soviet avionics, oxygen and de-icing systems; and metric materials throughout. The Tu-4 was a convincing look-alike, not a molecular duplicate.

Unlike Western aircraft of the period, the Tu-4 has no publicly reliable unit price, cost-per-flight-hour or programme total. Soviet defence accounting did not work in comparable terms, and the figures that circulate are estimates at best. What can be said is qualitative: the reverse-engineering effort consumed enormous national resources across hundreds of enterprises, and operating a fleet of four-radial-engined heavy bombers was expensive in fuel, spares and skilled manpower. We have marked every cost field as "not reliably recorded" rather than repeat unsupported numbers.

Beyond the baseline bomber and the nuclear-capable Tu-4A, the fleet spawned tanker and receiver conversions for refuelling trials, the Burlaki parasite-fighter tow aircraft, missile-carrier experiments and drone-director versions. In China the type was reworked most radically of all: the KJ-1 airborne-early-warning conversion mounted a large rotating radar dome and AI-20 turboprops, while the drone-carrier conversions lofted La-17 target drones. Not all of these entered service, but together they show how much life was wrung from the airframe.

The B-29 was not just an airframe; it was a flying electronics suite, and reproducing it forced the USSR to master radar, radio and remote-turret fire control at once. The Tu-4's Kobalt bombing-and-navigation radar was derived from the American AN/APQ-13 carried aboard the interned aircraft, while the remote-controlled gun system had to be re-engineered to drive Soviet 23 mm cannon.

This electronics dimension is often overlooked in the 'they copied a bomber' summary. Arguably the avionics were harder to reproduce than the aluminium, and the capabilities the USSR built to do it seeded a domestic industry in airborne radar and fire control that mattered long after the Tu-4 was gone.

An atomic bomb is only a deterrent if it can be delivered. The USSR tested its first nuclear device in August 1949, and the Tu-4A gave the growing arsenal its first credible air-delivery platform. In October 1951 a Tu-4 dropped the RDS-3 in a live test, demonstrating the full chain from aircraft to detonation.

For the first half of the 1950s, before missiles and longer-ranged bombers matured, the Tu-4 fleet was the visible edge of Soviet nuclear power. Its numbers and readiness were watched closely in the West, and its limitations - above all range - shaped how both superpowers thought about the balance of terror.

China received its first Tu-4s in 1953 and made them the core of the PLAAF's strategic bomber force at a moment when the country had almost no heavy-bomber capability of its own. Over the following decades the aircraft flew bombing, reconnaissance, transport and survey missions and became a familiar sight in Chinese service.

What makes the Chinese chapter remarkable is how far the aircraft was developed locally. When Soviet engine support became uncertain, China re-engined Tu-4s with domestically-supported AI-20 turboprops. It built the KJ-1 radar-dome early-warning conversion, and it turned others into drone carriers. The last Chinese Tu-4 was retired only in 1988, closing nearly four decades of service.

The Tu-4 entered service around 1949, and within less than a decade it was obsolete for front-line strategic work. The reason was the extraordinary pace of aeronautical progress in the 1950s. The swept-wing, jet-powered Tu-16 Badger offered far higher speed, and the turboprop Tu-95 Bear delivered the intercontinental range the Tu-4 had always lacked.

By the late 1950s the piston-engined Tu-4 had been pushed out of the first line at home, continuing in transport, tanker, trials and second-line duties. Its rapid eclipse is a measure of just how fast the technology moved - and of how sensible Stalin's original decision to copy rather than innovate had been, given that any home-grown 1940s piston bomber would have been overtaken just as quickly.

To understand the Tu-4 you have to understand what made the Boeing B-29 worth copying. It was the most sophisticated bomber of the Second World War: pressurised so its crew could operate in shirtsleeves at high altitude, defended by remote-controlled gun turrets aimed through an analogue fire-control computer, and able to carry heavy loads over very long ranges. It was the aircraft that dropped the atomic bombs on Hiroshima and Nagasaki.

Reproducing it handed the USSR not just a bomber but a template for modern aircraft manufacturing. If you want to see the original and the copy side by side, our B-29 Superfortress exhibit tells the American half of the story.

Because the Tu-4 lacked intercontinental range, the Soviet air force used it to explore ways of extending reach. Tu-4s trialled both wingtip-to-wingtip 'wing-to-wing' refuelling and hose-and-drogue methods, building the expertise that later made in-flight refuelling routine on Soviet jet bombers.

Stranger still were the Burlaki experiments, in which a Tu-4 towed a fighter behind it on a cable so the escort could conserve fuel and be 'delivered' fresh to the target area. It was a literal parasite-fighter concept, and while it never entered service, it typifies how the copied bomber became a platform for genuinely original Soviet ideas.

The Tu-4 carried a crew of about eleven, distributed like the B-29's through pressurised compartments connected by a crawl tunnel over the bomb bay. Up front sat the two pilots, the navigator, the bombardier in the glazed nose and the flight engineer, who had the demanding job of nursing four large radial engines and their turbo-superchargers.

Gunners operated the remote-controlled cannon turrets from sighting stations rather than sitting in the turrets themselves - an advanced arrangement for the 1940s that the Soviets had to reproduce and adapt to their own 23 mm guns. High-altitude, long-endurance flying in a piston bomber was cold, noisy and exhausting work, and crew endurance was a real operational limit.

NATO assigned the Tu-4 the reporting name 'Bull', following the convention of using a word beginning with 'B' for bombers. For Western intelligence the aircraft was a priority target of study: how many existed, where they were based, and above all whether they could reach North America.

The answer to that last question - not on a two-way mission - shaped Western threat assessments in the early 1950s and fed the debates about air defence, early-warning radar lines and the bomber balance that dominated the period. The Tu-4 was, in that sense, as important for the anxiety it created as for anything it ever did.

The airmen whose B-29s came down in the Soviet Far East in 1944 presented Moscow with a diplomatic problem. The USSR was neutral in the Pacific war and could not simply hand the crews or aircraft back to the United States without provoking Japan.

The solution was a piece of theatre: the interned Americans were moved to Soviet Central Asia and allowed to 'escape' across the border, a fiction that let the crews return home while the Soviet Union kept the bombers. The men went home; the aircraft became the seeds of the Tu-4.

Production totals for Soviet-era aircraft are notoriously slippery, and the Tu-4 is no exception. The figure most often cited is around 847 aircraft built between roughly 1949 and 1952, but you will also see totals quoted a little higher or lower depending on how derivatives, prototypes and conversions are counted.

We use '~847' and flag it as an estimate. What is not in doubt is the order of magnitude: this was a mass-produced bomber built in the high hundreds, not a handful of hand-made copies - which is exactly what made it strategically significant.

The Tu-4 did more than equip existing units; it helped define what Soviet Long-Range Aviation would become. Operating a large fleet of complex, four-engined pressurised bombers demanded new bases, new maintenance and logistics chains, new training, and new command structures for strategic air power.

By the time the jet Tu-16 and turboprop Tu-95 arrived, the USSR already had the institutional machinery to absorb them, thanks in large part to the Tu-4 years. The aircraft trained a generation of aircrew and ground staff in modern strategic operations.

The Tu-4 occupies an outsized place in aviation folklore precisely because its story is so cinematic: forced landings, an autocrat's order, a bolt-for-bolt copy, an atomic first. That drama is also why the myths - the copied patch above all - have proved so durable.

The honest version is more interesting than the legend. The Tu-4 was neither a mindless clone nor a Soviet original, but a vast act of technical translation that changed an industry and helped set the terms of the nuclear age. Retiring the myths does not diminish the aircraft; it lets the real achievement show.

One of the B-29's most advanced features was its defensive system: gunners aimed remote-controlled turrets from sighting stations, with an analogue computer correcting for range, lead and parallax. Reproducing this in the Tu-4 was a serious challenge, and the Soviets had to re-engineer the whole system to drive their heavier 23 mm cannon rather than the American machine guns.

Getting a 1940s electromechanical fire-control system to work with different weapons and Soviet-made components was arguably as demanding as any part of the airframe copy, and it pushed Soviet precision engineering and electronics forward in the process.

Standing beside a preserved Tu-4 at Monino or Datangshan, the first impression is simply how much it looks like a B-29 - the glazed nose, the slim aluminium fuselage, the four engine nacelles. Look closer and the differences emerge: the cannon barrels of the 23 mm turrets, the details of the engine cowlings, the Cyrillic stencilling.

These aircraft are the most tangible evidence of the whole story. They let you see, in a single glance, both the faithfulness of the copy and the quiet ways in which a Soviet aircraft asserted itself inside an American shape.