The programme began with a 1934 requirement for a heavy bomber able to strike deep behind enemy lines. Petlyakov’s brigade, working under Tupolev at TsAGI, produced the ANT-42 — a clean, all-metal four-engine monoplane far more advanced than the TB-3 it was meant to replace. Series development was later led by Iosif Nezval, who nursed the difficult aircraft through years of slow, hand-built production at Kazan’s Plant No. 124.

Every stage was dogged by trouble: the purges swept up key engineers, the specialised engines were in chronically short supply, and the airframe was complex and costly to build. The result was a bomber produced in tiny numbers — commonly cited as about 90 to 93 aircraft — that nonetheless became the only four-engine strategic bomber the Soviet aircraft industry ever fielded in the war. Everything else the USSR flew in the long-range role was either obsolete, twin-engined, or supplied by Lend-Lease.

High-altitude flight needs dense air fed to the engines, and in the mid-1930s no Soviet powerplant could supply enough on its own at 8,000–10,000 metres. The answer was the ATsN-2 — a Klimov M-100 engine mounted in the fuselage behind the crew, driving a centrifugal compressor that pumped pressurised air out to the four wing engines. With it, the prototype could operate at altitudes commonly cited near 11,000 metres, above the reach of most 1939 interceptors. But the unit added weight, complexity and a fire risk, and consumed internal volume. As dedicated high-altitude versions of the AM-35 became available, the fifth engine was deleted from production aircraft, and service Pe-8s topped out around 9,300 metres.

The Pe-8’s preferred AM-35A was also needed for the MiG-3, and fighter production came first. To keep bombers rolling, engineers fitted Charomskiy M-30 and M-40 aviation diesels, prized for their long range — but these proved dangerously unreliable, cutting out over enemy territory during the first Berlin raid of August 1941 and causing several losses. Many aircraft were retrofitted back to AM-35As, and when even those ran short, late Pe-8s received ASh-82 air-cooled radials of around 1,700–1,850 hp, which improved reliability at the cost of high-altitude performance. Few WWII bombers flew with such a bewildering variety of engines, and none of the fixes ever fully solved the problem.

The Pe-8 was a Soviet state product, built in tiny numbers during wartime and never sold on any open market, so a firm flyaway unit price simply does not exist in the public record. Nor is there any credible cost-per-flight-hour figure for the type — the Soviet Union did not publish such data, and Western analysts never had the fleet to study. What sources do agree on is that the Pe-8 was expensive and slow to build: complex, largely hand-assembled and chronically short of engines, which is a large part of why only around 90 aircraft were ever completed. Any precise cost figure attached to a Pe-8 should be treated as guesswork rather than fact.