[frankiep]

From the late fall of 1944 through the early spring of 1945, the Japanese launched more than 9,000 of these "fusen bakudan", or fire balloons, of which 300 were found or observed in the US.

http://www.japan-101.com/history/fire_balloons_or_balloon_bombs.htm

Just because you never heard of it doesn't mean it didn't happen. And just because they weren't very accurate doesn't mean that they were a very real threat.

[Alberta's Child]

The question was raised here in the context of a Japanese plan to bomb San Francisco. My point was that a balloon traveling several thousand miles across an ocean is hardly the kind of method one would use to carry out such a mission. You could have a near-perfect understanding of meteorology and launch 1,000 balloons every day from Japan -- and you would probably spend 500 years launching balloons without ever getting one to land in San Francisco.