26 U.S.C. § 2205

If the tax or any part thereof is paid by, or collected out of, that part of the estate passing to or in the possession of any person other than the executor in his capacity as such, such person shall be entitled to reimbursement out of any part of the estate still undistributed or by a just and equitable contribution by the persons whose interest in the estate of the decedent would have been reduced if the tax had been paid before the distribution of the estate or whose interest is subject to equal or prior liability for the payment of taxes, debts, or other charges against the estate, it being the purpose and intent of this chapter that so far as is practicable and unless otherwise directed by the will of the decedent the tax shall be paid out of the estate before its distribution.