For the fabrication of a bag-in-box (BIB) package, there is first provided a semifinished, collapsed BIB package wherein a flattened bag is placed within a collapsed box and secured to at least one of its inside surfaces by means of an adhesive. After erecting the package, the bag is inflated into close internal contact with the box by introducing a gas under pressure through a fitment attached to the bag and projecting outwardly of the box. The opposite ends of the box are closed with sets of foldable end flaps, with the aid of an adhesive. Preferably, the bag is further secured to the inside surfaces of a pair of opposed ones of the four bottom end flaps of the box. The opposed pair of bottom flaps are held folded out during the introduction of the pressurized gas into the bag, in order that the bottom end portion of the inflated bag may make neat contact with the inside surfaces of the box. There is also disclosed herein an apparatus for thus fabricating the BIB package.