With engines in the 2,000 to 2,500 hp range, twin-engined aircraft would have considerably more surplus power, allowing for much greater payloads.
In theory, the more powerful engine would take no longer to produce than a 1,000 hp design, it would simply be larger.
By the late 1930s, engines of this sort of power began to be seriously considered and the British and Germans drew up bomber designs based on them.

In the UK, Avro and Handley Page drew up proposals for a large bomber based on two Rolls-Royce Vulture engines.
The Vulture was a quartet of six-cylinder-long cylinder blocks connected to a common crankcase and crankshaft, to make a larger displacement X-block design.
As the bomber matured, problems with the Vulture became evident.
Contrary to hopes, bringing together two V-12 engines' "quartet" of cylinder banks onto one crankcase for a larger design led to all sorts of problems.
Development of the Avro Manchester pressed ahead but Handley Page was asked to adapt their HP.56 design for four smaller engines instead.
When the Manchester flew with all of the problems with the Vulture remaining, it too received a four-engine revision by its designer, Roy Chadwick.
The Avro Lancaster and Handley Page Halifax designs formed the backbone of RAF Bomber Command for the rest of the war.

In Germany, the Bomber A program in mid-1936 had led to the Heinkel He 177 A, powered by two Daimler-Benz DB 606 "power system" engines.
The 606 was a late-1930s attempt to use two Daimler-Benz DB 601 powerplants mated to a common gear reduction case to arrive at a 24-cylinder powerplant like the Vulture but using twin crankcases for the demanding German Heinkel He 119 high-speed reconnaissance design and the Messerschmitt Me 261 long range aircraft.
The pair of DB 601 component engines of the DB 606 were arranged in an inverted W-block layout, keeping each component engine as a discrete, "all up" engine as much as possible (except for the "final drive" to the propeller shaft) instead of the X-crankcase/cylinder layout.
Daimler-Benz was also working on a single-crankcase 24-cylinder design in parallel with the DB 606, creating the Daimler-Benz DB 604 as an X-engine design of their own, from 1939 through September 1942.
Like the Vulture, DB engineers found the DB 606, weighing in at a massive 1.5 tonnes apiece was mediocre, particularly when the airframe mounting them possessed an engine accommodation design that prevented adequate maintenance access and ventilation.

Production of the He 177A continued and in service with deficient powerplant installation, engine nacelle internal design and maintenance access, it was plagued by engine failures, overheating and fires while airborne, earning it the nickname "Flaming Coffin" by its crews.
Unlike the British and Ernst Heinkel, who complained in November 1938 over what Reichsmarschall Hermann Göring was calling welded-together engines by August 1942, the Luftwaffe was partly hesitant of the need for a truly "four-engined" Heinkel He 177B for level bombing, due to the November 1937-imposed dive bombing requirement on the existing He 177A, until Goering cancelled that requirement on September 15, 1942.
With engines in the 2,000 to 2,500 hp range, twin-engined aircraft would have considerably more surplus power, allowing for much greater payloads. In theory, the more powerful engine would take no longer to produce than a 1,000 hp design, it would simply be larger. By the late 1930s, engines of this sort of power began to be seriously considered and the British and Germans drew up bomber designs based on them.
In the UK, Avro and Handley Page drew up proposals for a large bomber based on two Rolls-Royce Vulture engines. The Vulture was a quartet of six-cylinder-long cylinder blocks connected to a common crankcase and crankshaft, to make a larger displacement X-block design. As the bomber matured, problems with the Vulture became evident. Contrary to hopes, bringing together two V-12 engines' "quartet" of cylinder banks onto one crankcase for a larger design led to all sorts of problems. Development of the Avro Manchester pressed ahead but Handley Page was asked to adapt their HP.56 design for four smaller engines instead. When the Manchester flew with all of the problems with the Vulture remaining, it too received a four-engine revision by its designer, Roy Chadwick. The Avro Lancaster and Handley Page Halifax designs formed the backbone of RAF Bomber Command for the rest of the war.
In Germany, the Bomber A program in mid-1936 had led to the Heinkel He 177 A, powered by two Daimler-Benz DB 606 "power system" engines. The 606 was a late-1930s attempt to use two Daimler-Benz DB 601 powerplants mated to a common gear reduction case to arrive at a 24-cylinder powerplant like the Vulture but using twin crankcases for the demanding German Heinkel He 119 high-speed reconnaissance design and the Messerschmitt Me 261 long range aircraft. The pair of DB 601 component engines of the DB 606 were arranged in an inverted W-block layout, keeping each component engine as a discrete, "all up" engine as much as possible (except for the "final drive" to the propeller shaft) instead of the X-crankcase/cylinder layout. Daimler-Benz was also working on a single-crankcase 24-cylinder design in parallel with the DB 606, creating the Daimler-Benz DB 604 as an X-engine design of their own, from 1939 through September 1942. Like the Vulture, DB engineers found the DB 606, weighing in at a massive 1.5 tonnes apiece was mediocre, particularly when the airframe mounting them possessed an engine accommodation design that prevented adequate maintenance access and ventilation.
Production of the He 177A continued and in service with deficient powerplant installation, engine nacelle internal design and maintenance access, it was plagued by engine failures, overheating and fires while airborne, earning it the nickname "Flaming Coffin" by its crews. Unlike the British and Ernst Heinkel, who complained in November 1938 over what Reichsmarschall Hermann Göring was calling welded-together engines by August 1942, the Luftwaffe was partly hesitant of the need for a truly "four-engined" Heinkel He 177B for level bombing, due to the November 1937-imposed dive bombing requirement on the existing He 177A, until Goering cancelled that requirement on September 15, 1942.
Bomber B