According to The Bell Helicopter Textron Story by David A. Brown,
D-200 is Bell number for the XV-3 convertiplane.
D-261 was an attack two seater for AAFSS contest (1964).
From Bell Aircraft by A J Pelletier (Putnam),
Model 212 is the UH-1N
Model 240 was Bell UTTAS contender (1972)
Model 250 is a project from Model 206 (1962)
Please note that Bell Model designations are different from Bell Design Numbers. I don't have info for D-212, D-240 or D-250 design numbers.