For the first time in the Battle, the Luftwaffe made a co-ordinated onslaught by all three of its air fleets (or Luftflotten) deployed from Norway to Brittany to stretch Fighter Command all along its line. This day saw the heaviest fighting of the whole battle.
The first blow came at 11.30am in the south east where a force of Junkers 87s escorted by Messerschmitt 109s came in between Dover and Dungeness to bomb Lympne and Hawkinge airfields. The little-used Lympne was put out of action for two days, the more important Hawkinge suffered little damage.
The next important event occurred much further north where, just after midday, radar detected a large force miles out to sea off the Firth of Forth. Responding vigorously to its first threat, 13 Group was able to scramble five squadrons to intercept about 100 Heinkel 111s, escorted by 70 Messerschmitt 110s, thirty miles from the coast. A heavy toll was taken at no cost to the defenders and, although some bombers made landfall, no damage was inflicted on military objectives.
At the same time ninety miles further south, about fifty unescorted Junkers 88s were heading for the Bomber Command base at Driffield in east Yorkshire. They were engaged by squadrons from both 12 and 13 Groups but, although several bombers were shot down, thirty aircraft got through to the target to cause heavy damage. However, this was the only success for Luftflotte 5 which had suffered so severely, losing one-eighth of its bomber force and one-fifth of its long-range fighters, that it never made another daylight attack during the entire battle. Fighter Command's losses were nil. The Germans had learned the harsh lesson that bombers on operations in daylight could not hope to survive without escort by Messerschmitt 109 single-seat fighters.
The day's fighting was far from over. An hour after the attack on Driffield, at 2.20pm, escorted dive bombers struck the aerodrome at Martlesham in Suffolk. At 3.30pm, a force of 100 bombers launched a heavy attack on the aircraft factories in Rochester, causing loss of output for several weeks, and also bombed Eastchurch. Hawkinge was hit for the second time that day.
The next big raid came in about two hours later when south coast radar stations plotted no fewer than seven strong formations numbering between two to three hundred aircraft approaching Hampshire and Dorset. In response, 10 and 11 Groups put up the largest force yet to counter a single operation by the Luftwaffe, about 150 Spitfires and Hurricanes. Strongly engaged over Portland and Portsmouth, the Germans took heavy losses and only managed to cause some damage to Middle Wallop fighter station. The action was less one-sided though than the morning's clash in the north as the defenders also suffered.
It was barely over when another threat, this time to 11 Group's left flank, materialised at a time when many squadrons had just landed to re-fuel after their second or third patrol of the day. Nevertheless, sufficient forces were available to challenge 60 or 70 incoming raiders. Victories were again scored by the defenders but the attacks were among the most effective that had yet been made. Croydon and West Malling airfields were hit, putting the latter out of action for several days, and two aircraft factories at Croydon were badly damaged.
15 August was a day of very hard fighting which had extended both sides almost to the limit. The Luftwaffe had flown over 2,000 sorties and had lost 75 aircraft. Fighter Command flew 974 sorties during the daytime and lost 34. At approximately 2 to 1, the ratio of fighter to bomber sorties by the Germans clearly demonstrated the scale of the effort needed to try to get through to targets. Their return on the day was very poor, considering the scale of attack. The damage caused was slight and no serious gaps had appeared in the defences.