An under-construction penthouse duplex in Boston will be going on the market for $40 million.

Located in the Four Seasons Private Residences One Dalton Street, Boston, the unit is the last of three penthouses, all situated on the 60th and 61st floors of the building, to go on the market. Developer Richard Friedman, president and chief executive of Cambridge-based Carpenter & Company, said a buyer entered into contract for one of the penthouses for “about $40 million” last year, and the second penthouse went into contract...