Rule 15(c)(3)(B) provides a further requirement for relating back an amended complaint that adds or changes a party: the newly added party knew or should have known that 'but for a mistake concerning the identity of the proper party, the action would have been brought against the party.' Fed. R. Civ. P. 15(c)(3)(B).
The bulk of authority from takes the position that the amendment of a 'John Doe' complaint --i.e., the substituting of real names for 'John Does' or 'Unknown Persons' named in an original complaint--does not meet the 'but for a mistake' requirement in 15(c)(3)(B), because not knowing the identity of a defendant is not a mistake concerning the defendant's identity. See Wilson v. United States, 23 F.3d 559, 563 (1st Cir. 1994); Barrow v. Wethersfield Police Dept., 66 F.3d 466, 469 (2d Cir. 1995), amended by 74 F.3d 1366 (2d Cir. 1996); W. Contracting Corp. v. Bechtel Corp., 885 F.2d 1196, 1201 (4th Cir. 1989); Jacobsen v. Osborne, 133 F.3d 315, 320 (5th Cir. 1998); Cox v. Treadway, 75 F.3d 230, 240 (6th Cir. 1996); Worthington v. Wilson, 8 F.3d 1253, 1256 (7th Cir. 1993); Powers v. Graff, 148 F.3d 1223, 1226-27 (11th ...