Japanese air raid bombs causing casualties first fell on Hong Kong on 8th December 1941? Apparently not.
Try 21st February 1939, when Japanese war planes bombed Shumchun district just across the border during an attack on a train travelling from Fanling to Lo Wu. At least two planes flew over into British territory and dropped bombs near the camp of the Middlesex regiment, destroyed the Chinese Maritime Customs building and dropped bombs near Lo Wu station, all approximately two miles from the HK Governor Sir Geoffry Northcote's residence in Fanling Lodge where the explosions were audible to the Governor and staff. Several fatalaties were reported.
Source: The Hong Kong Telegraph, pages 1 and 4, 22nd February 1939
Samples of other overseas newspapers reporting on the event:
The Argus (Melbourne, Australia), page 13, 22nd February 1939
The Palestine Post, 22nd February 1939
Briefly debated in the UK Parliament 27th February 1939