4 engined heavy bomber

PB4Y-1
The USAAF agreed to turn over a batch of B-24 Liberators to the USN, which designated them PB4Y-1s. Ther first of these saw combat with Marine reconnaissance squadron VMD-254 which was sent to Espiritu Santo in October 1942. Other Marine photo-reconnaissance units which saw action with PB4Y-1 were, VMD-154 in January 1943, and VMD-354 a year later. Photo-reconnaissance flights were done at about 22,000ft (6705m).
Dorr, p43, p47

PB4Y-1P
In late April 1943, the Navy's first photographic squadron equipped with B-24s arrived on Espiritu Santo. VD-1 immediately began photographic patrols throughouth the Solomons. The full name of this type of squadron was Fleet Air Photographic Reconnaissance Squadron, which was shortend to VD. The squadron was formed in the spring of 1942 and was equipped with B-24. B-24 aircraft assigned to such squadron were designated PB4Y-1P which were equipped with two 400 gallon fuel tanks in the forward bomb bays, and one of the two rear bays was equipped with 4 mapping cameras, while remainning bay could carry bombs.
Carey ge, p15

USN converted 65 Liberators for photo duty and established four squadrons — VD-1 (January 1943), VD-3 (February 1943), VD-4 (August 1943) and VD-5 (July 1944). I think USN needed reconnaissance to plan invasion and a lot of maps for amphibious operation. I understood how often USN did photo-reconnaissance missions for Iceberg Operation in Okinawa. USN organized three squadrons and prepared invasion in 1943. While IJN built air bases in the Marshall Islands.
Dorr, p44

B-17

Self-sealed fuel tank of B-17D

Layer	Material	Thick
No1	Neoprene	0.5mm
No2	Natural rubber	2mm
No3	Rubber cloth	0.5mm
No4	Vulcanized rubber	0.5mm
No5	Sponge rubber	4mm
No6	Leather	2mm
No7	Alminum	3mm

Japan captured B-17D in the end of 1941. IJN was surprised to see self sealed fuel tanks of B-17. So IJN checked the seal of B-17D. The table shows inner layers of the sealed tank. IJN bomber type 96 had made fire by bullets in 1937 in China. IJN flew Nells from Taipei, Ohmura, Saishu to Chaina mailand on 14, 15 and 16 August 1937. Kanoya Air Group decreased 10 working aircraft from 18 during 3 day missions. Kisarazu Air Group 8 from 20 aircraft. Then IJN studied self sealed fuel tank and fire extinguisher. But the study stopped because of weight.
Ikari, p147
Toyou bakugeki

B-17s were diffcult to shoot down. Zero almost shot enemies by 20mm guns except for B-17. The shells could not penetrate armor behind B-17 cockpit. While Betty was fragile for enemy fighters' 13mm shells. Crews of Betty called the aircraft type 1 lighter. By the way, Betty was not installed armor plates till 1943.
Genda, p238

AEW
VB-116 began ECM (Electronic Counter Measures) missions in March 1944, flying PB4Y-1 Liberators modified with installation of the AN/ARC-1 radar intercept receiver ‐ similiar to the US Army's SCR-587. The squadron three-ship sorties against Japanese radars on Truk.
Dorr, p44

USN planned B-17 as AEW operation. The designation was PB-1W.
PB-1W