The Nakajima B6N Tenzan (meaning "Heavenly Mountain", Allied reporting name: Jill) was the Imperial Japanese Navy's standard torpedo bomber for the final years of World War II. It was a development of the Nakajima B5N and replaced that aircraft in service. Although a highly-effective torpedo bomber, by the time it reached service, the U.S. Navy had already achieved air superiority over the Pacific, and the type never really had the opportunity to display its full potential.
The B6N1's combat debut was nothing short of disastrous. The Battle of the Philippine Sea had them operating in an environment where the U.S. had such air superiority that they failed to inflict any significant damage whatsoever, whilst taking massive losses from the U.S. Navy's new F6F Hellcat fighter. Following this debacle, the Navy ordered several changes to the design, most notably replacement of the NK7A Mamoru 11 engine with the Mitsubishi MK4T Kasei 25, resulting in the B6N2.
By this point, small improvements in the B6N's performance were amongst the least of the Japanese Navy's problems. When the new model became available in mid-1944, it had already lost most of its large carriers, and was becoming desperately short of experienced pilots. The vast majority of B6N2 operations therefore took place from land bases, and failed to achieve any major successes. They were extensively used in the Battle of Okinawa, where they were also used for kamikaze missions for the first time.  
                                                                                                                                        -- Wikipedia