September 9, 1942: The only Japanese attack on the US mainland
September 9, 1942, the I-25 B1 type Japanese submarine was cruising in an easterly direction raising its periscope occasionally as it neared the United States Coastline. The B1 type was the most numerous class of Japanese submarines. They were fast, long-ranged, and carried a seaplane behind watertight doors, which could be launched on a forward catapult. Japan had attacked Pearl Harbor less than a year ago and the Captain of the attack submarine knew that Americans were watching their coast line for ships and aircraft that might attack our country. Dawn was approaching; the first rays of the sun were flickering off the periscopes lens. Their mission; attack the west coast with incendiary bombs in hopes of starting a devastating forest fire.
If this test run were successful, Japan had hopes of using their huge submarine fleet to attack the eastern end of the Panama Canal to slow down shipping from the Atlantic to the Pacific. The Japanese Navy had a large number of I-400 submarines under construction, each capable of carrying three aircraft. Pilot Chief Warrant Officer Nobuo Fujita and his crewman Petty Officer Shoji Okuda were making last minute checks of their charts making sure they matched those of the submarine’s navigator.
The pilot of the aircraft checked his course and alerted his observer to be on the lookout for a fire tower which was on the edge of the wooded area where they were supposed to drop their bombs. These airplanes carried very little fuel and all flights were in and out without any loitering. The plane reached the shore line and the pilot made a course correction 20 degrees to the north. The huge trees were easy to spot and certainly easy to hit with the bombs. The fog was very wispy by this time. Johnson watched in awe as the small floatplane with a red meat ball on the wings flew overhead, the plane was not a bomber and there was no way that it could have flown across the Pacific, Johnson could not understand what was happening. He locked onto the plane and followed it as it headed inland.
The pilot activated the release locks so that when he could pickled the bombs they would release. His instructions were simple, fly at 500 feet, drop the bombs into the trees and circle once to see if they had started any fires and then head back to the submarine. Johnson could see the two bombs under the wing of the plane and knew that they would be dropped. He grabbed his communications radio and called the Forest Fire Headquarters informing them of what he was watching unfold. The bombs tumbled from the small seaplane and impacted the forests, the pilot circled once and spotted fire around the impact point. He executed an 180 degree turn and headed back to the submarine. There was no air activity, the skies were clear. The small float plane lined up with the surfaced submarine and landed gently on the ocean, then taxied to the sub. A long boom swung out from the stern. His crewman caught the cable and hooked it into the pickup attached to the roll over cage between the cockpits. The plane was swung onto the deck, The plane’s crew folded the wings and tail, pushed it into its hangar and secured the water tight doors. The I-25 submerged and headed back to Japan. Destroyer USS Patterson sank I-25 off the New Hebrides on 3 September 1943, almost exactly one year after the Oregon bombing.
This event, which caused no damage, marked the only time during World War II that an enemy plane had dropped bombs on the United States mainland.
http://delong.typepad.com/sdj/2012/09/liveblogging-world-war-ii-september-9-1942.html
= new reply since forum marked as read
I should pay closer attention to the other fine posts before tossing in my two cents. 
One of which was apparently in revenge for a butt full of cactus spines... 
http://www.militarymuseum.org/Ellwood.html
only thing students remember.
So, we've got it narrowed down to 'only air attack involving airplanes and not in Alaska (or Canada)'. Really an interesting story, and a pretty brave aircrew - imagine what it would have felt like taking off in your flimsy aircraft from a very transient launch pad to fly alone over a huge and pissed off country to deliver an attack that I have to imagine the flight and sub crews thought was a ridiculous idea in the first place...
