BD700 captain reported an near midair collision with a small aircraft on departure at bfi.

Attributes
ACN	1325633
Time
Date	201601
Local Time Of Day	1201-1800
Place
Locale Reference	BFI.Airport
State Reference	WA
Environment
Flight Conditions	VMC
Light	Daylight
Aircraft 1
Make Model Name	Global Express (BD700)
Operating Under FAR Part	Part 91
Flight Phase	Initial Climb
Route In Use	Vectors
Flight Plan	IFR
Aircraft 2
Make Model Name	Small Aircraft
Operating Under FAR Part	Part 91
Flight Phase	Initial Climb
Flight Plan	None
Person 1
Function	Captain Pilot Not Flying
Qualification	Flight Crew Multiengine Flight Crew Air Transport Pilot (ATP)
Experience	Flight Crew Last 90 Days 100 Flight Crew Total 21000 Flight Crew Type 4000
Events
Anomaly	Conflict NMAC
Miss Distance	Horizontal 100 Vertical 0

Narrative:

On takeoff; as we were cleaning the airplane up; PF looking out the heads up display (HUD) and ahead; PNF cleaning up flaps; the [small aircraft] appeared to drift into our departure path. We had been given notice of the closely matching tail number and that he had just departed. We had him in sight as he departed; but our speed rapidly caught up to them after takeoff.after we departed; he had drifted into our departure path and at 500 feet; we needed to turn 30 degrees to our right to avoid a collision. We passed within 100 feet (our wingspan) and at his altitude.tower was trying hard to get him to move; but their mic had broken at just that time and while they were replacing the mic another controller was also broadcasting for the [small aircraft] to move back left. We were hearing those transmissions. Tower sometimes splits controllers between the two parallel runways so we weren't hearing transmissions to the [small aircraft] until this occurred.as we went into our turn; the [small aircraft] started their turn to the left.in [all my] years of flying; this is as close as I've come to a collision. I was just cleaning up the airplane as PNF and when I looked up and out; the windshield was 3/4 full of another airplane's wing.

Original NASA ASRS Text

Title: BD700 Captain reported an NMAC with a small aircraft on departure at BFI.

Narrative: On takeoff; as we were cleaning the airplane up; PF looking out the Heads Up Display (HUD) and ahead; PNF cleaning up flaps; the [small aircraft] appeared to drift into our departure path. We had been given notice of the closely matching tail number and that he had just departed. We had him in sight as he departed; but our speed rapidly caught up to them after takeoff.After we departed; he had drifted into our departure path and at 500 feet; we needed to turn 30 degrees to our right to avoid a collision. We passed within 100 feet (our wingspan) and at his altitude.Tower was trying hard to get him to move; but their mic had broken at just that time and while they were replacing the mic another controller was also broadcasting for the [small aircraft] to move back left. We were hearing those transmissions. Tower sometimes splits controllers between the two parallel runways so we weren't hearing transmissions to the [small aircraft] until this occurred.As we went into our turn; the [small aircraft] started their turn to the left.In [all my] years of flying; this is as close as I've come to a collision. I was just cleaning up the airplane as PNF and when I looked up and out; the windshield was 3/4 full of another airplane's wing.

Data retrieved from NASA's ASRS site and automatically converted to unabbreviated mixed upper/lowercase text. This report is for informational purposes with no guarantee of accuracy. See NASA's ASRS site for official report.