Hello friendly historians!

I am reading a book about spacecraft when I came across the line "The airspace over the continental United States was never penetrated during World War II despite the efforts of the two largest air forces ever assembled" [1, pp 1].

I'm not super familiar with WWII, but as a casual observer I am not aware of any effort to penetrate the airspace of the continental US. Are there particular events or strategies that the Axis forces implemented to try to penetrate the airspace of mainland USA? If not, what do you think the above line means?

Thanks!

1. Brown, Charles D.. Elements of Spacecraft Design, American Institute of Aeronautics and Astronautics, 2000.