I am pretty sure that it was the B-25 Bombers of the 340th Bombardment Group that led the attack on Monte Cassino on February 15, 1944.
The 340th BG was made up of four squadrons: 486th, 487th, 488th and 489th.
Here is what the war diaries had to say about the mission:
486th
Crews knew at breakfast their target for the day – the already publicized monastery at Monte Cassino. Twelve ships of the 486th took off at 1006 hours and were over the abbey at 1059. They dropped 36 1000 lb bombs, with a majority of them landing in or near the buildings, causing fires and explosions. The buildings were obscured by smoke, and photographs do not reveal much. Crews felt the abbey was finito. A/A was negligible and all planes returned safely.
487th
The Abbey on Monte Cassino, which the Germans were shrewdly using as a Fortress was the target for
six of our ships today. The formation was over the target at 1059 hours, forming a good pattern and
dropped their 23x1000 bombs with excellent results, although several bombs fell short of the target and
landed on the slopes of the monastery. Several explosions and large fires were observed emitting from
the building. Ack-ack was heavy, slight and inaccurate. All ships and crews returned safely to the
base
488th
Mission to Cassino Abbey which the Germans had converted into a fortress. Allies have been threatening for days to blast it if the Germans didn't move out “because it was costing the lives of so many Americans!” Finally went at it today with B-17's, B-26's and '25s'. We had word later that the 321st and us working together had stolen the show from the '17s'.
[The History, as opposed to the Diary, also has the mission report and the flight manifest showing all 488th crew who were on the mission.]
489th
The Diary of the 489th only notes six planes sent on a mission to “Benedictine Monastery, Italy.”
Of course we know today that the German's were being true to their word and were not using the Monastery for military purposes. Once the Army Air Force reduced the building to rubble,however, the rubble produced an ideal cover for artillery observers and snipers.
The original War Diaries of the 340th BG can be found here:
http://57thbombwing.com/340th_History/340thHistory.php