With a cruising altitude of well over 30,000 feet and an economical cruising speed of 220 miles per hour few Japanese aircraft could hope to catch a B-29 at altitude, especially when it was in a shallow dive (a common tactic to transit the target area faster); the only Japanese antiaircraft weapon effective against a high-flying B-29, the Type 3 12 cm antiaircraft gun, was produced in small numbers (120). This did not make the B-29 absolutely invulnerable, however; many were lost to fighters and flak.
I would think the perceived effectiveness of the B-29's analog fire control system speaks for itself. On 27 January 1945, B-29-20-MO 42-65246 Irish Lassie ("A-Square 52") of the 497th Bombardment Group claimed to have shot down twelve Japanese fighters and was rammed by two more over Tokyo (one behind the left outboard engine, and another in the left portion of the tail), for a total of fourteen kills. In reality, only three confirmed kills were made, excepting the rammings, although it was still probably some kind of record for kills and claims by a single bomber on a single mission. The Irish Lassie made it back to base on Saipan on three engines and crash-landed; all the crewmen survived, but the aircraft was so severely damaged that it had to be written off.
Collier’s Magazine, 6 July 1946, Volume 118, no. 1;
SUPERFORTRESS Irish Lassie wheels in toward the coast of Japan at 2 o'clock on the afternoon of January 27, 1945, at 29,000 feet. The air over Japan is bright and freezing. Back at her base at Saipan, in the tropics, the day is hot. Below, a winter snow lies over Tokyo's streets.
Irish Lassie's crew is warm; they're flying pressurized over the target today and their oxygen masks hang down loosely to the left of their jaws. It is their eighth mission to Japan, though other B-29s from the 73rd Wing have been flying to Tokyo from Saipan for over two months.
In the blisters, in the tail, in the turrets, gunners squint anxiously through their Plexiglas, searching their arcs of sky. Someone shouts: "There they are—at 3 o'clock, high—God! About forty of 'em"
Over the coastline now, the fight begins. Forty enemy fighters pile into this first flight of twelve B-29s, from above, in front, the sides, from the tail. Central Fire Control Gunner Jim McHugh swings his upper turret, lets fly with his four fifties at the attackers from 12 o'clock. One Tojo plunges through.McHugh's tracers riddle him, but still he doesn't stop. At 400 miles an hour he plummets in at the Superfort and crash! In a suicide dive the Tojo rams Irish Lassie, smashing into the left wing, tearing out great hunks of the No. 1 engine nacelle and shearing off half the left aileron and flap. Her giant metal frame shudders, then recovers and heads on into her bombing run. The Tojo, spinning down out of control, blows up at 25,000 feet.
Despite the impact of the crash dive and the scores of hard-pressed fighter attacks, Bombardier Gage never budges from his sights. Delicately he twists the knobs, concentrates on the cross-hairs, whispers steadily into his throat-microphone. Deliberately his hand reaches out, closes over a toggle switch. "Bombs away! Bomb bay doors closed! Free to turn and for God's sake turn fast!" Twelve 500-pound bombs arch and tumble on their way down to the Musashino Aircraft Engine Plant, Tokyo.
Gage jumps back to his guns, blasts the canopy off an attacking Zeke, killing the pilot. Amidships, McHugh throws his four fifties onto another Zeke, attacking from above, and shatters him. Alone and isolated in the tail sits Charlie Mulligan. He's been having a busy time back there, fighting off the attackers. Already Mulligan has chalked up three certain kills since 2 o'clock. Now it is 2:20.
The Nips still come diving drunkenly in at Mulligan, plunging through his fire to within a few feet and then pulling wildly away. Right above him now one persistent devil flies. The Jap turns in for another attack; at 30 yards Mulligan lets fly with everything. The Jap never stops- but tears smack into the tail in a full power dive, smashing his plane to pieces on the giant fin. He almost takes Irish Lassie with him. His suicide dive tears off the entire left stabilizer, the whole left side of the tail compartment.
Up front in the pilot's seat, Avery feels the nose drop away violently and every man in the plane feels the terrible impact of this second ramming in less than half an hour. The B-29 falls off into a screaming dive. Avery pulls back hard on the control column and kicks the rudders. Nothing happens; the plane persists in her dive. Copilot Fox is on the controls too. He thinks he's just helping Avery out, with his extra strength, but it turns out he is actually flying the plane alone. There is just one thin strand of cable left to the elevators and that belongs to Fox.
The B29 continues to dive for 9,000 feet. Nobody expects to get out alive. Fox, using every ounce of his strength manages to level the ship out at around 20,000 feet and it is a miracle that their meager control cable doesn't snap.
The 20 Jap vultures now close in for the kill on the helpless cripple. But the blister gunners are still on the job. When it is all over, Irish Lassie's total kill is 13 [sic] Jap fighters.
Off the coast, Avery lets down to 17,000 feet and depressurize the cabin. Mulligan can't be raised over the interphone, so Leach and Meyer make their way aft to see if he is in trouble. Mulligan is—in the worst possible trouble. Bleeding and unconscious, his head between his knees, Mulligan lies crouched in the wrecked tail section. Twisted metal, broken guns, smashed equipment and splintered glass are piled on top of him. Working in shifts and with additional help from Gage and McHugh, Leach and Meyer extricate Mulligan and drag him inch by inch back towards the radar room. The temperature is subzero, the gale terrific, with the full draft and slipstream of the engines pounding directly in upon them.
The pilots are having their troubles. They have 1,500 miles to fly over ocean in a wrecked plane with scarcely more than a strand of cable to control it. In addition to their tail gunner being seriously wounded, their radar man, Klimczak, is badly shot up. In the radar compartments, Navigator Faubiom and Radio Operator Nellums are giving him first aid. He has been shot through the leg, back and arm, and he's bleeding profusely. They give him morphine and plasma, strip off his clothes and sprinkle his wounds with sulfa. They feed him full rich oxygen and wrap him in blankets.