Willow Run Project

You may have vehicles you cast enter the battlefield with X -1/-1 counters on them. If they
have a bomb ability, their bomb score is decreased
by X. Those vehicles cost 2(X) less mana in any combination of colours to cast.
Over 18,000 B-24 bomber's (above) were built during the war, 10 a day, on average. During the Willow Run project, they averaged a bomber every 63 minutes. Many were even built by car manufacturers.