high unit cost, Tallboys were used exclusively against high-value strategic targets that could not be destroyed by other means. When it was found that the Lancaster could be modified to carry a bomb larger than the Tallboy, Wallis produced the even larger Grand Slam bomb. Operations June – August 1944 Saumur rail tunnel The sole operational north-south route on the Loire. Nineteen Tallboy-equipped and six conventionally equipped Lancasters of 617 Squadron attacked on the night of 8/9 June 1944. 617 Squadron were guided on to the target by 83 Squadron Pathfinder Force. This was the first use of the Tallboy bomb and the line was destroyed—one Tallboy bored through the hillside and exploded in the tunnel about below, completely blocking it. No aircraft were lost during the raid. Operation Crossbow Crossbow was the code name for measures to counter the German V-1 flying bomb ("buzz bomb" or "doodlebug") and V-2 rocket weapons. Tallboys were used by the British to destroy several missile sites. 19 June 1944 – Watten The nearest Tallboy dropped by 617 Squadron landed from the target, a heavily fortified V-2 launch site under construction The bunker was rendered useless. 24 June 1944 – Wizernes The target was a V-2 assembly and launch site linked with the Watten site. Several Tallboy hits undermined the foundations but did not penetrate the dome. The bunker was abandoned. 25 June 1944 – Siracourt V-1 bunker Lancasters of 617 Squadron scored three direct hits with Tallboys without loss. 4 July 1944 – Saint-Leu-d'Esserent 617 Squadron used seventeen Lancasters with Tallboys, supported by one Mosquito and one Mustang, in an attempt to collapse the limestone roof of the caves used as storage depots. Aircraft from No 5 Group followed up with bombs. 6 July 1944 – Mimoyecques Attack on V-weapon targets. Damage was unknown at the time, and efforts continued. In September, allied ground forces found galleries blocked with earth and debris where Tallboys had hit one of the shafts. The V-weapon was revealed to be the V-3 cannon. 17 July 1944 – Wizernes 16 Lancasters, led by a Mosquito and a Mustang, bombed Wizernes – three Lancasters managed to drop Tallboys (one caused the dome to shift out of alignment, two others blocked the entrance). 27 July 1944 – Watten One Tallboy hit the target but did not penetrate the structure. 31 July 1944 – Rilly La Montagne Both ends of the