The inventive blanket is an inflatable underbody convective thermal blanket whereon the head and the upper body of a patient may lie. The blanket is in the shape of a cross with a truncated lower vertical portion and an upper head portion onto which the head of the patient lies. The blanket has two horizontal arm portions that extend from the main body portion of the blanket and onto which the arms of the patient may be positioned. At each of the arm portions there is a flap that may be used to cover the arm of the patient. An inlet port is provided in an upper corner at the head portion of the blanket. The heated air input to the blanket is directed by a plurality of sealed strips to flow in respective desired directions around the arm portions and the lower truncated body portion of the blanket. Sets of apertures are provided along an inner sealed periphery that defines a T-shaped uninflated area onto which the torso of the patient lies. There are more apertures at the inner corners of the inflatable structure defined by the uninflated area to enable a greater outflow of heated air at the lower truncated body portion of the blanket. All inner and outer corners of the blanket are rounded to facilitate the air flow.