Why Few RAF Fortresses?
Heavies were a UK invention, 1935/36, to extend inland Britain's traditional mode of warfare, which was to visit mass destruction upon our enemy's infrastructure. Long the task of RN, blockade and bombardment, RAF was to be a Force of aerial Monitors. From April,1938 much of UK engineering industry was turned to producing Warwick, Manchester, Stirling, Halifax. France's nationalised aero-industry was riddled with Communist Unions, who presumed Heavies would be used against the Workers' Paradise, so French Govt. funded its 4-motor from Consolidated, LB-30.
1936/37-US had no need for any of this, but funded a batch of B-17B as maritime patroller: nose-gun-only to strafe and to deal with rising carrier-borne interceptors; "walking" release of small bombs, Norden-aimed, to scatter the Imperial Navy approaching Philippines, to be disposed of by USN. Boeing mispriced it and faced bankruptcy; UK ordered 20 B-17C (for cash money): that plus a French order on Douglas for 240 DB-7 (Boston I), subbed to Boeing in April,1940, paid by UK, kept them alive.
The Task of deep inland bombardment given to RAF in 1935/36 was not given to USAAF until mid-1940. FDR funded Very Heavy B-29/B-32, to be preceded by B-17E/F/G, took over (to be) B-24, and converted non-aero resources - Rosie the Rivetter - into a shadow production system (Govt. Plants at Renton, Wichita, Douglas and Vega for B-17s, Ford's Willow Run phenomenon for B-24) and made the human investment to operate thousands...maybe from 1943. From March,1941 Lend/Lease was on offer to the King's Forces (US, remember, was neutral!), but UK chose not to request large quantities of distant B-17 (D, in build: E, onway), because UK Heavies would arrive sooner (Stirling already here, Halifax imminent) and tried instead to have Lancasters licence-built in US. From 8 December,1941 it suited both US and UK to turn US/Canadian bauxite into US designs in US and into UK designs in UK/Canada.
RAF did take c.200 B-17E/F/G, confined to Coastal and to ECM; the King's Forces did operate more B-24s (2,443) than Stirlings (2,369), largely in Commonwealth/overseas Units. Reasons are your choice of: ops. were complex enough just mixing up Forces of multiple-Brit types; no Norden sight; supply chain must be wholly separate for MilSpec bits from BS bits. Mull gently on Targeting Policy: UK/US agreed the Combined Bomber Offensive at the highest policy level, then left daily detail to the men at the coal face. FDR could have taken very close interest, say in "area" as opposed to "panacea" targeting, if UK had been using his kit.