The Navy’s 1937 requirement asked for a land-based bomber with the range of a flying boat but the speed of a modern aircraft — and it capped the design at two engines. Kiro Honjo, who had studied advanced structures in Germany, knew what that combination demanded. Every kilogram spent on armour plate or rubber-lined self-sealing tanks was a kilogram of fuel not carried, so those items were deleted. The wing was built as an integral “wet” structure, its interior sealed and filled with fuel.

The trade delivered the range the Navy wanted, and in 1941–42 that reach let the Betty strike where no one expected it. Yet the very tanks that gave it range had no way to seal a puncture or smother a fire. From 1943 the Navy tried to claw back some protection — adding covers over the tanks on later G4M1s, and finally a properly armoured G4M3 in 1944 — but by then Allied fighters and radar-directed guns were everywhere, and only about sixty G4M3s were completed before the war ended.

Power came from two Mitsubishi Kasei (“Mars”) fourteen-cylinder radials of roughly 1,530 hp each, giving a top speed around 428 km/h. Defensively the early Betty relied on a scatter of 7.7 mm machine guns and a single 20 mm cannon in the tail. Allied pilots quickly learned that the guns could be endured and the fuel could not — the surest kill was a burst into the wing roots. Later marks added heavier tail armament and, eventually, tank protection, but the fundamental vulnerability was baked into the original bargain.

From 1944 the G4M2e was modified to carry the Ohka — a small, rocket- or jet-boosted piloted bomb designed to be released near an Allied ship and flown into it by its pilot. The Betty had to lug the heavy, draggy weapon most of the way to the target, becoming slower and even more vulnerable in the process. On the first mass sortie, on 21 March 1945, the entire formation of mother ships was intercepted and destroyed before any Ohka could be launched. It was a grim illustration of how far the Betty’s lack of protection had caught up with it by the war’s end.

The G4M was a wartime Japanese state product, built by Mitsubishi for the Imperial Japanese Navy and never sold on an open market. No dependable flyaway unit price or cost-per-flight-hour figure survives in the public record, and wartime Japanese accounting was disrupted by the collapse of 1945. What is well documented is the scale of production — roughly 2,435 airframes across all variants — which made the Betty the Navy’s most numerous land-based bomber. Any precise dollar figure attached to a G4M today should be read as an estimate, not a sourced fact.