Top 10+ Extreme Warplane Modifications of the Second World War

Vast swarms of aircraft were built during the Second World War as combatants constantly tried to seize an advantage.

With so many aircraft, it made sense to modify an existing aircraft rather than come up with something new. What follows are ten such modifications. This includes crewed missiles, flying tanks, ludicrous firepower, and fighters grafted together to form new conjoined machines. Some were brilliant, some were appalling failures, a few were outstandingly successful, and all shed light on a terrifying innovation race with the highest possible stakes:

10: US Heavy Bomber fighter conversions: YB-40 and XB-41

During the Second World War, converting fighters into bombers was a fairly commonplace occurrence, but examples of bombers that became fighters were far rarer. Those there were tended to be small, fast bombers such as the de Havilland Mosquito; the idea of a heavy bomber becoming a fighter seems absurd.

Yet the USAAF attempted exactly that, converting examples of both the B-17 Flying Fortress and B-24 Liberator into escort fighters. Only one example of the XB-41 Liberator, modified to carry fourteen .50-calibre machine guns and over 14,000 rounds of ammunition, was built. Although testing proved problematic, the B-17 conversion proved slightly more successful.

The Boeing YB-40 was equipped with up to thirty defensive guns, though it typically carried fourteen or sixteen. Armament was mostly .50 calibre machine guns in various configurations, though 40-mm cannons were also tested. Twenty-five examples were built, and the aircraft was tested on combat missions over Europe.

No aircraft has ever flown with such a formidable gun-based defensive armament. Unfortunately, this made the aircraft so heavy and slow that it couldn’t keep up with the bombers it was supposed to protect. After participating in just ten missions, the entire project was abandoned.

9: Grumman XF4F-3S ‘Wildcatfish’

Various land-based fighters were converted to become seaplanes during the Second World War. A few were very impressive, like the Spitfire floatplanes; some of the others were, to put it kindly, somewhat underwhelming. One such was the floatplane derivative of Grumman’s spectacularly successful F4F Wildcat, which was developed in 1942.

Inspired by the Japanese use of floatplane fighters from remote island bases in the Pacific, the float manufacturer Edo was contracted to convert a standard F4F-3 fighter to operate from water. As well as the floats, the aircraft required a ventral fin as well as auxiliary fins on the tail to maintain stability.

The Wildcat was a slow fighter to start with and slower than its primary foe, the Mitsubishi Zero. The addition of two hefty floats and their supporting struts chopped the maximum speed down to a pretty woeful 241mph. It was, therefore, obvious the floatplane could only be used where fighter opposition was unlikely.

Despite this, 100 sets of Wildcat floats were produced in great haste, but only one floatplane conversion, nicknamed the ‘Wildcatfish’, was ever constructed. The ability of US forces to rapidly capture or construct remote island airbases saw the requirement for a floatplane fighter effectively evaporate.   

8: Aerial minesweepers

Magnetic mines posed a significant threat to shipping: 79 ships were lost to mines by the end of 1939 alone. The British responded swiftly with a modified Wellington bomber fitted with a 48-foot-diameter balsa-wood ring containing aluminium coils that emitted magnetic impulses when charged by an electrical current.

The electrical ring mimicked the magnetic signature of a ship to detonate mines below the surface. Aircraft were much faster than conventional minesweepers, allowing them to clear large areas at high speed. The process was hazardous, however, requiring crews to fly at 130 mph just 35 to 60 feet above the water.

At such low altitudes, the aircraft were also liable to damage, or even destruction, from the mines they were detonating. Nonetheless, minesweeping Wellingtons proved highly successful, keeping the Thames Estuary clear of magnetic mines for the duration of the war and able to respond quickly to suspected minefields or to clear specific areas.

The Germans also employed the same technology, using the Bv 138 flying boat and Ju 52/3m transport. However, German minesweeping operations were hampered by Allied fighters; this particular famous photo shows a Ju 52 under attack by a Hawker Typhoon. Surviving German minesweepers were used postwar to clear remaining minefields.

7: High altitude combat: Ju 86 and Spitfire

The Ju 86 was an unexceptional medium bomber that was nearing the end of its operational life in 1939 when Junkers modified it into the Ju 86P high-altitude aircraft, featuring a hugely extended wingspan, a pressurised crew compartment, and turbocharged diesel engines, enabling it to operate at 40,000 feet.

40 Ju 86Ps conversions were built and operated with impunity over Britain, conducting reconnaissance and nuisance bombing. The improved Ju 86R variant followed with an even greater wingspan and enhanced engines, allowing for flight at 47,000ft. Although never causing much material damage, the uninterceptable Ju 86 raids proved infuriating.

High-altitude Spitfires Mk VI and VII (pictured) were developed with extended wings and pressurised cockpits, but by this point, lightened Spitfire IXs were coaxed high enough to make an interception. The highest known combat of the war occurred when a Spitfire IX damaged a Ju 86 at 44,000ft in August 1942, off the coast of Egypt.

Although that particular Ju 86 was not been destroyed, the realisation that the RAF was capable of intercepting the high-flying bombers resulted in their withdrawal from missions over the British Isles. Two Ju 86s were subsequently lost to Spitfires in the Mediterranean, causing Ju 86 operations in the West to cease altogether.

6: Fieseler Fi 103R ‘Reichenburg’

Desperate times call for desperate measures, but few measures were ever as desperate as the Fieseler Fi 103R. Consisting of a V-1 cruise missile with a cockpit crammed in behind the 900kg warhead, the pilot was expected to point the aircraft at its target before bailing out.

The standard V-1 was fast and cheap, but woefully inaccurate, proving able to (sometimes) hit a city-sized target but not much good against anything smaller. The addition of a pilot was hoped to make the weapon viable for attacking ships or other tactical targets.

Despite appearances, the Fi 103R was not intended as a suicide weapon. However, the prospect of survival was rated as “most unlikely.” Bailing out was complicated by the pulsejet engine intake directly behind the canopy, and it was predicted that fewer than 1% of pilots would escape with their lives.

Some prospective pilots were given training in gliders. Still, the almost certain death sentence offered by the Fi 103R was realised to be too insane even for the Nazis and Hitler was persuaded to cancel it. An unknown number of Fi 103Rs were manufactured, and one survives at the Swiss Military Museum.

5: Rotabuggy and A-40

In addition to modifications to existing aircraft, the Second World War also saw the conversion of ground-based vehicles into aircraft. The most promising was likely the Rotabuggy, developed by Austrian-born helicopter pioneer Raoul Hafner, consisting of a jeep equipped with an unpowered rotor to allow it to be towed by an aircraft.

Tests proved encouraging, and in September 1944, following release from an Armstrong Whitworth Whitley tug aircraft, the Rotabuggy flew for ten minutes at 65mph, its flying qualities described as “highly satisfactory”. However, by this point, transport gliders were already carrying jeeps into battle, removing any pressing need for the Rotabuggy. 

The Soviet Union went one stage further, deciding that flying tanks behind enemy lines to support partisans or airborne troops would be a splendid idea. Initially, the focus was on designing a large glider to carry a tank, but designer Oleg Antonov thought converting the tank itself into a glider was a better idea. 

Therefore, a T-60 light tank was fitted with jettisonable biplane wings and tail. Tested just once in September 1942, the tank glider was successfully flown and driven back to its base after landing. However, the lack of a suitably powerful tug aircraft to pull the heavy tank glider doomed the project. 

4: B-25 Gunships

The B-25 was already being used as a versatile medium bomber when a field modification altered it into a terrifically effective gunship and led to it becoming the most heavily armed attack aircraft of the war. This all derived from the work of one man, the appropriately named Major Paul ‘Pappy’ Gunn.

Gunn had fitted Douglas A-20s with extra nose guns recovered from wrecked fighters, and these had proved so successful he was asked to convert a squadron of B-25s along similar lines. Modified in Townsville, Australia, with eight forward firing .50 calibre machine guns in the forward fuselage, the modified B-25s were called ‘Commerce Destroyers’.

At the Battle of the Bismarck Sea, eleven Commerce Destroyers proved utterly deadly against Japanese ships, utilising their insane firepower to strafe before skip-bombing 500-pound bombs into the ships. The muzzle flashes of the nose guns were so intense that Japanese sailors believed the attackers were hit and on fire.

The astonishing success of the extemporised B-25 gunships led directly to the appearance of the factory-built B-25G and H gunships. Featuring a giant 75mm cannon, a modified version of a Sherman tank’s main gun, the B-25H backed this up with ten forward-firing .50 calibre machine guns. 

3: Mistel

Unlike the unfortunate occupant of the Fi 103R, the pilot of the Mistel (‘Misteltoe’) had a decent chance of survival as he was provided with a fully functional fighter to make good his escape. This composite aircraft was an intriguing scheme to utilise obsolete Ju 88 bombers to attack high-value targets.

Comprising a Bf 109 or Fw 190 mounted on the back of an explosive-laden Ju 88, the Mistel composite would fly directly at the target before the fighter detached to escape, leaving the uncrewed bomber to continue directly into, and hopefully destroy, the target.

Operational Mistels replaced the Ju 88’s cockpit with a shaped explosive charge of nearly two tonnes and a prominent detonator. In June 1944, Mistels attacked the invasion fleet at Courseulles-sur-Mer, France, but no damage was caused despite participating pilots reporting hits. It appears that a decoy battleship hulk had successfully lured the attackers.

Mistels were later used to attack bridges in an attempt to halt the Soviet advance into Germany. The effects, however, were negligible and delayed the Red Army only slightly. Ultimately, the Mistel failed due to the absence of a means to steer the bomber component accurately onto its target.

2: North American P-82 Twin Mustang

Likely the most extreme aircraft modification to enter production, the Twin Mustang was the result of an almost absurdly logical process to design a longer-ranged fighter than the standard Mustang to escort long-distance B-29 Superfortress raids against Japan. If one Mustang was insufficient, why not combine two?

Using the experimental XP-51F as a basis, two lengthened Mustang fuselages and outer wings were mated by a new wing centre section and horizontal tail. Each cockpit contained full controls so the pilots could take it in turns to fly the aircraft, reducing fatigue on missions expected to last many hours. 

Although it first flew in June 1945, the P-82 saw no wartime service. Performance was, however, sensational, and an early P-51B flew nonstop from Hawaii to New York, a distance of 5051 miles in 14 hours and 32 minutes —a distance record for piston-engine fighters that remains unbroken.

Political pressure saw later aircraft fitted with Allison engines rather than Rolls-Royce Merlins, and performance suffered. New jet fighters sidelined the P-82 to the night-fighting role, where its excellent endurance proved invaluable. In June 1950, a Twin Mustang achieved the first US aerial victory of the Korean War.

1: TB-3 and I-16 ‘Zveno’

Derived from pre-war experiments with parasite fighters carried aloft by a mothership (the ‘Aviamatka’), the Soviet Navy’s Zveno (‘Link’) composite bomber consisted of a Tupolev TB-3 heavy bomber carrying two manned and bomb-equipped Polikarpov I-16 fighters under its wings.

The combination allowed the I-16s to be carried to targets much further than they could usually reach, as well as permitting them to deliver heavier bombs than they could usually carry. The vulnerable TB-3 would release the two fighters at a safe distance, and all three would fly home separately.