The X3DH protocol was developed by Moxie Marlinspike and Trevor Perrin.
The underlying "Triple DH" key agreement was proposed by Caroline Kudla and Kenny Paterson in [8], extending the earlier "Double DH" (aka "Protocol 4") key agreement from Simon Blake-Wilson et al [9]. Using signatures in combination with implicitly-authenticated key agreement has been discussed in works like [10] and [11].
Thanks to Mike Hamburg for discussions about identity binding and elliptic curve public keys.
Thanks to Nik Unger and Matthew Green for discussions about deniability.
Thanks to Matthew Green, Tom Ritter, Joseph Bonneau, and Benedikt Schmidt for editorial feedback.