Finally, my own list of Bell Aircraft "D-for design" numbers. Any corrections and additions are most welcome!
D-6 design name of Model 32 / XP-77 fighter
D-13 competitor project of XP-55 (same as Model 16?)
D-? WWII era tailsitter project
D-22 a "Bell Design 22" appears in the Sara Clark collection at the US National Archives
D-35 delta-wing single seat jet fighter project
D-37 swept-wing version of X-1 (Design 37D is said to have been the X-2)
D-39 P-63 interceptor
D-79 single-place rotor plane
D-82A Transport rotor plane; gross weight: 23,000 lbs.
D-82B rotor plane for rescue; gross weight: 23,000 lbs.
D-89 a "Bell Design 89" appears in the Sara Clark collection at the US National Archives
D-90 fixed-wing, supersonic derivative of the X-5, itself a derivative of the Messerschmitt P.1101 (1950)
D-100 single-engine research rotor plane (developed D-79)
D-104 projected civil version of Model 61 (Navy HSL)
D-109 CONVERT-O-PLANE VTOL fighter project (1951)
D-110? possibly a development of D-109
D-116 projected civil version of Model 61 (Navy HSL)
D-118 CONVERTIPLANE (VERTIBURNER?) project submitted to US Navy; two-place single engine aircraft for observation and reconnaissance
D-137? possibly a fighter
D-138? twin-boom fighter project?
D-139 VERTIBURNER VTOL day fighter weapon system (1955) (preliminary design was larger and more conventional)
D-143? twin-boom fighter project?
D-171 expermental hypersonic aircraft; design submitted for the X-15 competition
D-174 very early X-14-like study
D-179 very early X-14-like study
D-181 4-duct ducted propeller assault transport study
D-182A twin duct testbed
D-182C twin duct testbed; trailrotor T-37 COIN conversion project (1956)
D-188 early tilt-jet fighter project developed from D-139 project but with conventional-type thrust
D-188A experimental tilt-jet fighter project ("XF-109"/"F3L") (WS-318A?) (Model 2000)
D-190 VTOL study, rescue concept with tilt-ducted propellers on each wing, based on D-181/182 research (1959)
D-190/C-130 VTOL with wing-tip ducted fans, was to be fitted under a specially modified C-130 launch aircraft
D-190A D-190 development, no details
D-190B D-190 development, Multi SVVP with two propellers turning in the ring canals
D-191 ?
D-196 D-190 development, no details
D-200 turbine-powered helicopter project (or the design number for the XV-3 CONVERTIPLANE?)
D-207 cargo convertiplane (1953)
D-212? convertiplane project?
D-215? helicopter proposal?
D-216 derivative of HSL-1 as 25-seat US Army (attack?) helicopter (also commercial?)
D-217G? no details
D-218 cargo helicopter of twin side-by-side rotor scheme, would have been powered by two R-1820-34 engines (1956)
D-220 observation-reconnaissance version of XV-3 with T58 turbine (General Electric) (1955)
D-222 air rescue convertiplane with multi-engines (1955)
D-223 vertical takeoff transport convertiplane for internal and external cargo, with 4 GE XT58 turbines (1956)
D-224 air-rescue convertiplane with 3 General Electric ST-112 turbines (1956)
D-225 air rescue convertiplane with 2 Allison T56 turbines (1956)
D-230 AERIAL JEEP, a VTOL aircraft project with four foldable rotors. Not built. (also found as "FLYING JEEP") (1957)
D-232 tilt wing rotor feasibility study (1957)
D-238 derivative of HSL-1
D-240? convertiplane project?
D-242 target activation convertiplane (1959)
D-243 one- to two-ton cargo convertiplane (1959)
D-244 utility command convertiplane (1959)
D-245 forerunner of 'COBRA'
D-246/A medium transport convertiplane powered by 4 Lycoming T55-L-5 turbines (1959/60)
D-250 convertiplane project?
D-252 military VTOL transport: squared fuselage section, 2x T64 turbines; entered into Tri-Service competition (1960)
D-252A NATO version of D-252 (1961)
D-255 full-size mock-up of 'IROQUOIS WARRIOR'
D-? 'AIR SCOOTER' ground effects machine (GEM), a one-man unit with a hover height of 2-1/2 inches, cruising speed of 25 mph
D-? 'TRAILROTOR' concept, full-scale testbed project
D-261 attack two-seater for AAFSS contest (1964)
D-262 IROQUOIS WARRIOR attack two-seater for AAFSS contest; the project originated the AH-1 COBRA
D-266 proprotor VTOL research aircraft composite pre-design study proposal; light tactical transport Developed in response to Army's Composite Aircraft Program; requirement w/ 2x 2850shp T64-GE / 30-seat short-haul civil aircraft; project ended early 1969 due to lack of funding (1965)
D-267 first design study for a full-scale tilt-rotor demonstrator; significantly smaller than the D-266, and oriented toward commercial rather than military operation (1968)
D-270/A design study for a 60,000-lbs. 4-jet (proprotor) heavy transport with wing-tip tilt-rotors (1969)
D-271 proprotor proof of concept aircraft design (1969)
D-272 folding proprotor proof of concept aircraft design (1969)
D-276 light twin helicopter project
D-280 project of a wide-bodied COBRA
D-292 light composite materials version of 222 (ACAP program)
D-294 design name of X-16
D-300 CARA (Combat Air Rescue) civilian tilt-rotor transport with conventional tail, forerunner of Model 301/XV-15A (1967)
D-301 military VTOL tilt rotor operational transport with conventional tail, forerunner of Model 301/XV-15A (1971)
D-302 NASA conceptual design study tilt rotor (1971)
D-303 civilian VTOL transport with H-tail, extended fuselage and jet mounted on top; using the same amount of fuel as a helicopter, able to fly twice as fast and twice as far; design was for ideal use on the North Sea. Projected to be operational by 1990 (1980)
D-305 COD/VOD tilt rotor proposal (1972)
D-306 business helicopter, 6 to 10 passengers; mock-up built
D-310 HX study for Navy tilt rotor aircraft (1974)
D-311 HX study for Marine tilt rotor (1974)
D-312 large civilian tilt rotor VTOL commercial (passenger) transport with H-tail and square fuselage section (-100, -45, -21) (1974)
D-313 tilt rotor STOL commercial transport (-100, -45) (1974)
D-314A "Tiltrotor Army Spectrum study; attack VTOL designed for ground support and FAC duties;
AH-1 type cockpit on XV-15 general arrangement (1975)"
D-314B Tiltrotor Army Spectrum study (1975)
D-314C Tiltrotor Army Spectrum study. A light military troop transport VTOL (1975)
D-314D Tiltrotor Army Spectrum study. observation VTOL designed for duties similar to OV-1 Mohawk; OV-1 type cockpit on XV-15 general arrangement (1975)
D-314E Tiltrotor Army Spectrum study. An uprated military cargo transport version of the D-314C and which is technically a medium lift transport (1975)
D-314F Tiltrotor Army Spectrum study. Heavily armed attack VTOL with armored cockpit designed for ground attack duties (2 different designs) (1975)
D-314G Tiltrotor Army Spectrum study. military VTOL which carries an onboard side looking airborne radar and other sensor units for OV-1-type operations (1975)
D-315A/B Tilt rotor HXM aircraft conceptual design study (1975)
D-316 Air Force combat rescue mission tilt rotor aircraft (1976)
D-317 ASH tilt rotor aircraft (1976)
D-318 LAMPS tilt rotor aircraft (1976)
D-319 Tilt rotor conceptual designs (resulting from Navy R.F.Q./I.) [D-319A, A-1, A(P), B, B(Q), C, D] (1978)
D-320 Tilt rotor conceptual designs for the U.S.A.F. rescue mission analysis (1978)
D-321 tilt-rotor conceptual VTOL design for US Navy V/STOL-A (pre-R.F.Q. activity) (1978-80)
D-322 Heavy lift quad rotor (resulting NASA inquiry) (1979)
D-323 V-tailed assault transport VTOL for US Marines with folding wing and tail (1979)
D-324 Navy Type A V/STOL (1979)
D-325 project for Navy (1979)
D-326 'CLIPPER' commercial tiltrotor (passenger transport) (1980)
D-327 VTOL aircraft with 10.5 m rotors (diameter) and a capacity for 24 paratroopers (1980-1981)
D-329 'BAT' research VTOL, proposal for Army's 1984 LHX light helicopter program ; could not meet the weight requirements. Rotor diameter: 4.87 meters (1984)
D-340 'POINTER' tilt-rotor UAV
D-400 more refined development of X-14 with retractable gear
D-600 several tilt-rotor projects; final one was six- to nine-passenger commercial tilt-rotor aircraft (Model 609)
D-604? tilt rotor aircraft project?
D-1007 huge nuclear-powered double-deck twin-rotor helicopter project
D-2001 two-seater tilt-rotor
D-2004 D-190 development with tip turbine drive ducts
D-2005 assault transport for USMC, forerunner design of X-22 (summer 1959)
D-2014 similar project with clamshell freight loading doors in tail
D-2020 smaller corporative D-2022 derivative
D-2021A smaller corporative D-2022 derivative
D-2021B development of D-2001
D-2022 formal proposal derivated from D-2005, USMC VTOL Assault Transport System Requirement/ US Army ASR 3-60 VTOL Transport System Study; forerunner of X-22 (1960)
D-2064 military VTOL transport development of D-2022; joint design with Lockheed to meet TS-152 for a tri-service STOL
D-2064A improved D-2064
D-2127 X-22A tilt-duct VTOL research plane
D-2149-1 SK-3 'CARABAO' ground effect machine
D-2161 no data
D-2172 alternate proposal for X-22 program?
D-2278 V/STOL aircraft technology (Air Force Systems Command)
D-2285 ULV – ultra light V/STOL
D-2289 USN ASW V/STOL aircraft study
D-2300 lift cruise fan V/STOL ASW
D-2301 cruise fan V/STOL ASW
D-2332 armed X-22A proposal
D-2410 USAF Tactical Air Command VTOL aircraft
D-2424 X-22A test project (Naval Air Systems Command)
D-2447 light anti-submarine warfare (ASW) aircraft
