The Dornier Do 23/G-2 — The Forgotten Bomber That Captured a Submarine (1939)

By the dawn of the 1930s, the Luftwaffe’s rebirth was still a secret ambition rather than a reality, but Germany’s aircraft industry was already preparing for it. Among the pioneers was Dornier Flugzeugwerke, which began developing what would become the Reich’s first post–World War I heavy bomber — the Do 11. Conceived in secrecy and under civilian cover, the Do 11 marked the first step in Germany’s slow return to strategic air power.

However, the aircraft’s early promise faded quickly. The Do 11 suffered from structural weaknesses, poor handling, and underpowered engines. In a continuous effort to salvage the design, Dornier introduced a series of improvements — first the Do 13, and then the Do 23, which entered production in 1934. Despite its ungainly appearance and limited performance, the Do 23 served as a vital stepping stone toward more capable bombers such as the Do 17, He 111, and Ju 86.

By 1936, with the new generation of fast, all-metal bombers taking over, the Do 23 was already obsolete. Yet Dornier engineers refused to let the design die. Believing the aircraft could still serve as a reliable long-range platform, they undertook one final modernization effort — the Do 23/G-2.

This last variant received numerous refinements: a cleaner, more aerodynamic fuselage, reinforced wings, and most importantly, new BMW IX engines, each producing 950 horsepower compared to the 750 of the earlier BMW VI. These upgrades pushed the top speed from 260 km/h to nearly 295 km/h — still no match for the sleek new twin-engine bombers, but enough to justify limited production. Somewhere between 20 and 30 examples were completed by late 1936, serving briefly with bomber squadrons before being replaced and reassigned to training and auxiliary roles.

At the outbreak of the Second World War, the aging Do 23s were largely forgotten relics of an earlier era. Most were relegated to training schools, while a handful were converted for experimental purposes — including chemical dispersal tests and anti-mine operations with large degaussing rings fitted beneath their fuselages.

But the aircraft’s final chapter came in October 1939, during a routine navigation exercise over the North Sea — an incident that would become the Do 23’s only recorded combat “victory.”

A formation of three Do 23/G-2s from a northern training squadron took off from an airfield near Hamburg, flying a triangular route over Helgoland Bight. About 50 miles northwest of the island, they spotted a submarine on the surface — a British S-class boat that had been forced to surface due to mechanical failure. The bombers, unarmed with bombs but carrying full belts of machine-gun ammunition, immediately attacked. Their 7.92-mm MGs stitched the water around the submarine, causing no real damage but terrifying its exposed crew.

Out of ammunition and unable to dive, the British commander ordered his men to raise a white flag — surrendering to the three lumbering bombers circling overhead. The lead aircraft radioed the nearby Helgoland naval base, which hastily dispatched an elderly World War I–era torpedo boat to take the surrender. For nearly two hours, the three Dornier crews circled the crippled submarine like guardian vultures until the German ship arrived and escorted the captured sub and its crew back to port.

The “Helgoland Incident,” as it became known among Dornier test pilots, was never widely publicized, but it remains one of the strangest episodes of the early war — a slow, obsolete bomber design from the early 1930s claiming the surrender of a modern Royal Navy submarine.

After 1940, the last Do 23s were scrapped or cannibalized for parts, their brief moment of accidental glory already fading into obscurity. Yet in that one bizarre encounter over the grey waters of the North Sea, the Dornier Do 23/G-2 earned a peculiar place in aviation history — the only aircraft ever to capture an enemy submarine without dropping a single bomb.