I landed 4x 37mm hits directly on a B-25 taking off once, and despite a hit on the cockpit and a direct hit to the engine, the only visible damage was an oil leak (engine still running).
It's possible the 37mm is off, but I think more than likely there are bugs in the damage models where things don't react properly when hit. Look at the long-standing dud 30mm bug where the cowl guns would soak up tons of damage without spreading it any further. They just fixed that. There might be more.