In between the world wars, the Germans were prohibited from developing military aircraft and vehicles.
As if.
They developed mail planes that could be quickly converted to bombers, and airliners that could also serve as troop transports.
The British were painfully oblivious to these dual-purpose vehicles, to the point that Germans were blatantly building tanks and calling them milk trucks and school busses.
“Oh, did I say this was a school bus?” says a grinning German diplomat, leaning on a tank. “I meant to say milk truck.”
“Looks good to me,” said Prime Minister Chamberlain. “Two pints of milk, please?”